Give userspace a way to instruct the kernel to install a pidfd into the
usermode helper process. This makes coredump handling a lot more
reliable for userspace. In parallel with this commit we already have
systemd adding support for this in [1].
We create a pidfs file for the coredumping process when we process the
corename pattern. When the usermode helper process is forked we then
install the pidfs file as file descriptor three into the usermode
helpers file descriptor table so it's available to the exec'd program.
Since usermode helpers are either children of the system_unbound_wq
workqueue or kthreadd we know that the file descriptor table is empty
and can thus always use three as the file descriptor number.
Note, that we'll install a pidfd for the thread-group leader even if a
subthread is calling do_coredump(). We know that task linkage hasn't
been removed due to delay_group_leader() and even if this @current isn't
the actual thread-group leader we know that the thread-group leader
cannot be reaped until @current has exited.
Link: https://github.com/systemd/systemd/pull/37125 [1]
Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
include/linux/coredump.h | 1 +
2 files changed, 56 insertions(+), 4 deletions(-)
diff --git a/fs/coredump.c b/fs/coredump.c
index 9da592aa8f16..403be0ff780e 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -43,6 +43,9 @@
#include <linux/timekeeping.h>
#include <linux/sysctl.h>
#include <linux/elf.h>
+#include <linux/pidfs.h>
+#include <uapi/linux/pidfd.h>
+#include <linux/vfsdebug.h>
#include <linux/uaccess.h>
#include <asm/mmu_context.h>
@@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
#define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
/* Define a reasonable max cap */
#define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
+/*
+ * File descriptor number for the pidfd for the thread-group leader of
+ * the coredumping task installed into the usermode helper's file
+ * descriptor table.
+ */
+#define COREDUMP_PIDFD_NUMBER 3
static int core_uses_pid;
static unsigned int core_pipe_limit;
@@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
case 'C':
err = cn_printf(cn, "%d", cprm->cpu);
break;
+ /* pidfd number */
+ case 'F': {
+ /*
+ * Installing a pidfd only makes sense if
+ * we actually spawn a usermode helper.
+ */
+ if (!ispipe)
+ break;
+
+ /*
+ * Note that we'll install a pidfd for the
+ * thread-group leader. We know that task
+ * linkage hasn't been removed yet and even if
+ * this @current isn't the actual thread-group
+ * leader we know that the thread-group leader
+ * cannot be reaped until @current has exited.
+ */
+ cprm->pid = task_tgid(current);
+ err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
+ break;
+ }
default:
break;
}
@@ -493,7 +523,7 @@ static void wait_for_dump_helpers(struct file *file)
}
/*
- * umh_pipe_setup
+ * umh_coredump_setup
* helper function to customize the process used
* to collect the core in userspace. Specifically
* it sets up a pipe and installs it as fd 0 (stdin)
@@ -503,12 +533,33 @@ static void wait_for_dump_helpers(struct file *file)
* is a special value that we use to trap recursive
* core dumps
*/
-static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
+static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
{
struct file *files[2];
struct coredump_params *cp = (struct coredump_params *)info->data;
int err;
+ if (cp->pid) {
+ struct file *pidfs_file __free(fput) = NULL;
+
+ pidfs_file = pidfs_alloc_file(cp->pid, 0);
+ if (IS_ERR(pidfs_file))
+ return PTR_ERR(pidfs_file);
+
+ /*
+ * Usermode helpers are childen of either
+ * system_unbound_wq or of kthreadd. So we know that
+ * we're starting off with a clean file descriptor
+ * table. So we should always be able to use
+ * COREDUMP_PIDFD_NUMBER as our file descriptor value.
+ */
+ VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
+
+ err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
+ if (err < 0)
+ return err;
+ }
+
err = create_pipe_files(files, 0);
if (err)
return err;
@@ -598,7 +649,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
}
if (cprm.limit == 1) {
- /* See umh_pipe_setup() which sets RLIMIT_CORE = 1.
+ /* See umh_coredump_setup() which sets RLIMIT_CORE = 1.
*
* Normally core limits are irrelevant to pipes, since
* we're not writing to the file system, but we use
@@ -637,7 +688,7 @@ void do_coredump(const kernel_siginfo_t *siginfo)
retval = -ENOMEM;
sub_info = call_usermodehelper_setup(helper_argv[0],
helper_argv, NULL, GFP_KERNEL,
- umh_pipe_setup, NULL, &cprm);
+ umh_coredump_setup, NULL, &cprm);
if (sub_info)
retval = call_usermodehelper_exec(sub_info,
UMH_WAIT_EXEC);
diff --git a/include/linux/coredump.h b/include/linux/coredump.h
index 77e6e195d1d6..76e41805b92d 100644
--- a/include/linux/coredump.h
+++ b/include/linux/coredump.h
@@ -28,6 +28,7 @@ struct coredump_params {
int vma_count;
size_t vma_data_size;
struct core_vma_metadata *vma_meta;
+ struct pid *pid;
};
extern unsigned int core_file_note_size_limit;
--
2.47.2Hi,
On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote:
> Give userspace a way to instruct the kernel to install a pidfd into the
> usermode helper process. This makes coredump handling a lot more
> reliable for userspace. In parallel with this commit we already have
> systemd adding support for this in [1].
>
> We create a pidfs file for the coredumping process when we process the
> corename pattern. When the usermode helper process is forked we then
> install the pidfs file as file descriptor three into the usermode
> helpers file descriptor table so it's available to the exec'd program.
>
> Since usermode helpers are either children of the system_unbound_wq
> workqueue or kthreadd we know that the file descriptor table is empty
> and can thus always use three as the file descriptor number.
>
> Note, that we'll install a pidfd for the thread-group leader even if a
> subthread is calling do_coredump(). We know that task linkage hasn't
> been removed due to delay_group_leader() and even if this @current isn't
> the actual thread-group leader we know that the thread-group leader
> cannot be reaped until @current has exited.
>
> Link: https://github.com/systemd/systemd/pull/37125 [1]
> Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
> include/linux/coredump.h | 1 +
> 2 files changed, 56 insertions(+), 4 deletions(-)
>
> diff --git a/fs/coredump.c b/fs/coredump.c
> index 9da592aa8f16..403be0ff780e 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -43,6 +43,9 @@
> #include <linux/timekeeping.h>
> #include <linux/sysctl.h>
> #include <linux/elf.h>
> +#include <linux/pidfs.h>
> +#include <uapi/linux/pidfd.h>
> +#include <linux/vfsdebug.h>
>
> #include <linux/uaccess.h>
> #include <asm/mmu_context.h>
> @@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
> #define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
> /* Define a reasonable max cap */
> #define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
> +/*
> + * File descriptor number for the pidfd for the thread-group leader of
> + * the coredumping task installed into the usermode helper's file
> + * descriptor table.
> + */
> +#define COREDUMP_PIDFD_NUMBER 3
>
> static int core_uses_pid;
> static unsigned int core_pipe_limit;
> @@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> case 'C':
> err = cn_printf(cn, "%d", cprm->cpu);
> break;
> + /* pidfd number */
> + case 'F': {
> + /*
> + * Installing a pidfd only makes sense if
> + * we actually spawn a usermode helper.
> + */
> + if (!ispipe)
> + break;
> +
> + /*
> + * Note that we'll install a pidfd for the
> + * thread-group leader. We know that task
> + * linkage hasn't been removed yet and even if
> + * this @current isn't the actual thread-group
> + * leader we know that the thread-group leader
> + * cannot be reaped until @current has exited.
> + */
> + cprm->pid = task_tgid(current);
> + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
> + break;
> + }
> default:
> break;
> }
>
I tried this change with Apport: I took the Ubuntu mainline kernel build
https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to
mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied
these three patches on top. Then I modified Apport to take the
additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result
is the coredump failed as long as there was `-F%F` on
/proc/sys/kernel/core_pattern:
```
coredump: 7392(divide-by-zero): |/usr/share/apport/apport pipe failed
```
Did I do something wrong? Do I miss additional patches?
--
Benjamin Drung
Debian & Ubuntu Developer
On Fri, Apr 25, 2025 at 01:31:56PM +0200, Benjamin Drung wrote:
> Hi,
>
> On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote:
> > Give userspace a way to instruct the kernel to install a pidfd into the
> > usermode helper process. This makes coredump handling a lot more
> > reliable for userspace. In parallel with this commit we already have
> > systemd adding support for this in [1].
> >
> > We create a pidfs file for the coredumping process when we process the
> > corename pattern. When the usermode helper process is forked we then
> > install the pidfs file as file descriptor three into the usermode
> > helpers file descriptor table so it's available to the exec'd program.
> >
> > Since usermode helpers are either children of the system_unbound_wq
> > workqueue or kthreadd we know that the file descriptor table is empty
> > and can thus always use three as the file descriptor number.
> >
> > Note, that we'll install a pidfd for the thread-group leader even if a
> > subthread is calling do_coredump(). We know that task linkage hasn't
> > been removed due to delay_group_leader() and even if this @current isn't
> > the actual thread-group leader we know that the thread-group leader
> > cannot be reaped until @current has exited.
> >
> > Link: https://github.com/systemd/systemd/pull/37125 [1]
> > Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
> > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > ---
> > fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
> > include/linux/coredump.h | 1 +
> > 2 files changed, 56 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/coredump.c b/fs/coredump.c
> > index 9da592aa8f16..403be0ff780e 100644
> > --- a/fs/coredump.c
> > +++ b/fs/coredump.c
> > @@ -43,6 +43,9 @@
> > #include <linux/timekeeping.h>
> > #include <linux/sysctl.h>
> > #include <linux/elf.h>
> > +#include <linux/pidfs.h>
> > +#include <uapi/linux/pidfd.h>
> > +#include <linux/vfsdebug.h>
> >
> > #include <linux/uaccess.h>
> > #include <asm/mmu_context.h>
> > @@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
> > #define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
> > /* Define a reasonable max cap */
> > #define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
> > +/*
> > + * File descriptor number for the pidfd for the thread-group leader of
> > + * the coredumping task installed into the usermode helper's file
> > + * descriptor table.
> > + */
> > +#define COREDUMP_PIDFD_NUMBER 3
> >
> > static int core_uses_pid;
> > static unsigned int core_pipe_limit;
> > @@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> > case 'C':
> > err = cn_printf(cn, "%d", cprm->cpu);
> > break;
> > + /* pidfd number */
> > + case 'F': {
> > + /*
> > + * Installing a pidfd only makes sense if
> > + * we actually spawn a usermode helper.
> > + */
> > + if (!ispipe)
> > + break;
> > +
> > + /*
> > + * Note that we'll install a pidfd for the
> > + * thread-group leader. We know that task
> > + * linkage hasn't been removed yet and even if
> > + * this @current isn't the actual thread-group
> > + * leader we know that the thread-group leader
> > + * cannot be reaped until @current has exited.
> > + */
> > + cprm->pid = task_tgid(current);
> > + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
> > + break;
> > + }
> > default:
> > break;
> > }
> >
>
> I tried this change with Apport: I took the Ubuntu mainline kernel build
> https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to
> mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied
> these three patches on top. Then I modified Apport to take the
> additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result
> is the coredump failed as long as there was `-F%F` on
I have no clue what -F%F is and whether that leading -F is something
specific to Apport but the specifier is %F not -F%F. For example:
> cat /proc/sys/kernel/core_pattern
|/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %F
And note that this requires the pipe logic to be used, aka "|" needs to
be specified. Without it this doesn't make sense.
On Fri, 2025-04-25 at 13:57 +0200, Christian Brauner wrote:
> On Fri, Apr 25, 2025 at 01:31:56PM +0200, Benjamin Drung wrote:
> > Hi,
> >
> > On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote:
> > > Give userspace a way to instruct the kernel to install a pidfd into the
> > > usermode helper process. This makes coredump handling a lot more
> > > reliable for userspace. In parallel with this commit we already have
> > > systemd adding support for this in [1].
> > >
> > > We create a pidfs file for the coredumping process when we process the
> > > corename pattern. When the usermode helper process is forked we then
> > > install the pidfs file as file descriptor three into the usermode
> > > helpers file descriptor table so it's available to the exec'd program.
> > >
> > > Since usermode helpers are either children of the system_unbound_wq
> > > workqueue or kthreadd we know that the file descriptor table is empty
> > > and can thus always use three as the file descriptor number.
> > >
> > > Note, that we'll install a pidfd for the thread-group leader even if a
> > > subthread is calling do_coredump(). We know that task linkage hasn't
> > > been removed due to delay_group_leader() and even if this @current isn't
> > > the actual thread-group leader we know that the thread-group leader
> > > cannot be reaped until @current has exited.
> > >
> > > Link: https://github.com/systemd/systemd/pull/37125 [1]
> > > Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
> > > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > > ---
> > > fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
> > > include/linux/coredump.h | 1 +
> > > 2 files changed, 56 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/fs/coredump.c b/fs/coredump.c
> > > index 9da592aa8f16..403be0ff780e 100644
> > > --- a/fs/coredump.c
> > > +++ b/fs/coredump.c
> > > @@ -43,6 +43,9 @@
> > > #include <linux/timekeeping.h>
> > > #include <linux/sysctl.h>
> > > #include <linux/elf.h>
> > > +#include <linux/pidfs.h>
> > > +#include <uapi/linux/pidfd.h>
> > > +#include <linux/vfsdebug.h>
> > >
> > > #include <linux/uaccess.h>
> > > #include <asm/mmu_context.h>
> > > @@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
> > > #define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
> > > /* Define a reasonable max cap */
> > > #define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
> > > +/*
> > > + * File descriptor number for the pidfd for the thread-group leader of
> > > + * the coredumping task installed into the usermode helper's file
> > > + * descriptor table.
> > > + */
> > > +#define COREDUMP_PIDFD_NUMBER 3
> > >
> > > static int core_uses_pid;
> > > static unsigned int core_pipe_limit;
> > > @@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> > > case 'C':
> > > err = cn_printf(cn, "%d", cprm->cpu);
> > > break;
> > > + /* pidfd number */
> > > + case 'F': {
> > > + /*
> > > + * Installing a pidfd only makes sense if
> > > + * we actually spawn a usermode helper.
> > > + */
> > > + if (!ispipe)
> > > + break;
> > > +
> > > + /*
> > > + * Note that we'll install a pidfd for the
> > > + * thread-group leader. We know that task
> > > + * linkage hasn't been removed yet and even if
> > > + * this @current isn't the actual thread-group
> > > + * leader we know that the thread-group leader
> > > + * cannot be reaped until @current has exited.
> > > + */
> > > + cprm->pid = task_tgid(current);
> > > + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
> > > + break;
> > > + }
> > > default:
> > > break;
> > > }
> > >
> >
> > I tried this change with Apport: I took the Ubuntu mainline kernel build
> > https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to
> > mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied
> > these three patches on top. Then I modified Apport to take the
> > additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result
> > is the coredump failed as long as there was `-F%F` on
>
> I have no clue what -F%F is and whether that leading -F is something
> specific to Apport but the specifier is %F not -F%F. For example:
>
> > cat /proc/sys/kernel/core_pattern
> |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %F
>
> And note that this requires the pipe logic to be used, aka "|" needs to
> be specified. Without it this doesn't make sense.
Apport takes short option parameters. They match the kernel template
specifiers. The failing pattern:
$ cat /proc/sys/kernel/core_pattern
|/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -F%F -- %E
Once I drop %F Apport is called without issues:
$ cat /proc/sys/kernel/core_pattern
|/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E
--
Benjamin Drung
Debian & Ubuntu Developer
On Fri, Apr 25, 2025 at 02:03:34PM +0200, Benjamin Drung wrote:
> On Fri, 2025-04-25 at 13:57 +0200, Christian Brauner wrote:
> > On Fri, Apr 25, 2025 at 01:31:56PM +0200, Benjamin Drung wrote:
> > > Hi,
> > >
> > > On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote:
> > > > Give userspace a way to instruct the kernel to install a pidfd into the
> > > > usermode helper process. This makes coredump handling a lot more
> > > > reliable for userspace. In parallel with this commit we already have
> > > > systemd adding support for this in [1].
> > > >
> > > > We create a pidfs file for the coredumping process when we process the
> > > > corename pattern. When the usermode helper process is forked we then
> > > > install the pidfs file as file descriptor three into the usermode
> > > > helpers file descriptor table so it's available to the exec'd program.
> > > >
> > > > Since usermode helpers are either children of the system_unbound_wq
> > > > workqueue or kthreadd we know that the file descriptor table is empty
> > > > and can thus always use three as the file descriptor number.
> > > >
> > > > Note, that we'll install a pidfd for the thread-group leader even if a
> > > > subthread is calling do_coredump(). We know that task linkage hasn't
> > > > been removed due to delay_group_leader() and even if this @current isn't
> > > > the actual thread-group leader we know that the thread-group leader
> > > > cannot be reaped until @current has exited.
> > > >
> > > > Link: https://github.com/systemd/systemd/pull/37125 [1]
> > > > Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
> > > > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > > > ---
> > > > fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
> > > > include/linux/coredump.h | 1 +
> > > > 2 files changed, 56 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/fs/coredump.c b/fs/coredump.c
> > > > index 9da592aa8f16..403be0ff780e 100644
> > > > --- a/fs/coredump.c
> > > > +++ b/fs/coredump.c
> > > > @@ -43,6 +43,9 @@
> > > > #include <linux/timekeeping.h>
> > > > #include <linux/sysctl.h>
> > > > #include <linux/elf.h>
> > > > +#include <linux/pidfs.h>
> > > > +#include <uapi/linux/pidfd.h>
> > > > +#include <linux/vfsdebug.h>
> > > >
> > > > #include <linux/uaccess.h>
> > > > #include <asm/mmu_context.h>
> > > > @@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
> > > > #define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
> > > > /* Define a reasonable max cap */
> > > > #define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
> > > > +/*
> > > > + * File descriptor number for the pidfd for the thread-group leader of
> > > > + * the coredumping task installed into the usermode helper's file
> > > > + * descriptor table.
> > > > + */
> > > > +#define COREDUMP_PIDFD_NUMBER 3
> > > >
> > > > static int core_uses_pid;
> > > > static unsigned int core_pipe_limit;
> > > > @@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> > > > case 'C':
> > > > err = cn_printf(cn, "%d", cprm->cpu);
> > > > break;
> > > > + /* pidfd number */
> > > > + case 'F': {
> > > > + /*
> > > > + * Installing a pidfd only makes sense if
> > > > + * we actually spawn a usermode helper.
> > > > + */
> > > > + if (!ispipe)
> > > > + break;
> > > > +
> > > > + /*
> > > > + * Note that we'll install a pidfd for the
> > > > + * thread-group leader. We know that task
> > > > + * linkage hasn't been removed yet and even if
> > > > + * this @current isn't the actual thread-group
> > > > + * leader we know that the thread-group leader
> > > > + * cannot be reaped until @current has exited.
> > > > + */
> > > > + cprm->pid = task_tgid(current);
> > > > + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
> > > > + break;
> > > > + }
> > > > default:
> > > > break;
> > > > }
> > > >
> > >
> > > I tried this change with Apport: I took the Ubuntu mainline kernel build
> > > https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to
> > > mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied
> > > these three patches on top. Then I modified Apport to take the
> > > additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result
> > > is the coredump failed as long as there was `-F%F` on
> >
> > I have no clue what -F%F is and whether that leading -F is something
> > specific to Apport but the specifier is %F not -F%F. For example:
> >
> > > cat /proc/sys/kernel/core_pattern
> > |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %F
> >
> > And note that this requires the pipe logic to be used, aka "|" needs to
> > be specified. Without it this doesn't make sense.
>
> Apport takes short option parameters. They match the kernel template
> specifiers. The failing pattern:
>
> $ cat /proc/sys/kernel/core_pattern
> |/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -F%F -- %E
>
> Once I drop %F Apport is called without issues:
>
> $ cat /proc/sys/kernel/core_pattern
> |/usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E
Youm must have CONFIG_DEBUG_VFS=y enabled where we trample the pidfs
file we just allocated. It's a debug only assert. I've removed it now
and pushed it to vfs-6.16.coredump. Can you either try with that or
simply unset CONFIG_DEBUG_VFS and retest.
On Fri, 2025-04-25 at 18:49 +0200, Christian Brauner wrote:
> On Fri, Apr 25, 2025 at 02:03:34PM +0200, Benjamin Drung wrote:
> > On Fri, 2025-04-25 at 13:57 +0200, Christian Brauner wrote:
> > > On Fri, Apr 25, 2025 at 01:31:56PM +0200, Benjamin Drung wrote:
> > > > Hi,
> > > >
> > > > On Mon, 2025-04-14 at 15:55 +0200, Christian Brauner wrote:
> > > > > Give userspace a way to instruct the kernel to install a pidfd into the
> > > > > usermode helper process. This makes coredump handling a lot more
> > > > > reliable for userspace. In parallel with this commit we already have
> > > > > systemd adding support for this in [1].
> > > > >
> > > > > We create a pidfs file for the coredumping process when we process the
> > > > > corename pattern. When the usermode helper process is forked we then
> > > > > install the pidfs file as file descriptor three into the usermode
> > > > > helpers file descriptor table so it's available to the exec'd program.
> > > > >
> > > > > Since usermode helpers are either children of the system_unbound_wq
> > > > > workqueue or kthreadd we know that the file descriptor table is empty
> > > > > and can thus always use three as the file descriptor number.
> > > > >
> > > > > Note, that we'll install a pidfd for the thread-group leader even if a
> > > > > subthread is calling do_coredump(). We know that task linkage hasn't
> > > > > been removed due to delay_group_leader() and even if this @current isn't
> > > > > the actual thread-group leader we know that the thread-group leader
> > > > > cannot be reaped until @current has exited.
> > > > >
> > > > > Link: https://github.com/systemd/systemd/pull/37125 [1]
> > > > > Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
> > > > > Signed-off-by: Christian Brauner <brauner@kernel.org>
> > > > > ---
> > > > > fs/coredump.c | 59 ++++++++++++++++++++++++++++++++++++++++++++----
> > > > > include/linux/coredump.h | 1 +
> > > > > 2 files changed, 56 insertions(+), 4 deletions(-)
> > > > >
> > > > > diff --git a/fs/coredump.c b/fs/coredump.c
> > > > > index 9da592aa8f16..403be0ff780e 100644
> > > > > --- a/fs/coredump.c
> > > > > +++ b/fs/coredump.c
> > > > > @@ -43,6 +43,9 @@
> > > > > #include <linux/timekeeping.h>
> > > > > #include <linux/sysctl.h>
> > > > > #include <linux/elf.h>
> > > > > +#include <linux/pidfs.h>
> > > > > +#include <uapi/linux/pidfd.h>
> > > > > +#include <linux/vfsdebug.h>
> > > > >
> > > > > #include <linux/uaccess.h>
> > > > > #include <asm/mmu_context.h>
> > > > > @@ -60,6 +63,12 @@ static void free_vma_snapshot(struct coredump_params *cprm);
> > > > > #define CORE_FILE_NOTE_SIZE_DEFAULT (4*1024*1024)
> > > > > /* Define a reasonable max cap */
> > > > > #define CORE_FILE_NOTE_SIZE_MAX (16*1024*1024)
> > > > > +/*
> > > > > + * File descriptor number for the pidfd for the thread-group leader of
> > > > > + * the coredumping task installed into the usermode helper's file
> > > > > + * descriptor table.
> > > > > + */
> > > > > +#define COREDUMP_PIDFD_NUMBER 3
> > > > >
> > > > > static int core_uses_pid;
> > > > > static unsigned int core_pipe_limit;
> > > > > @@ -339,6 +348,27 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm,
> > > > > case 'C':
> > > > > err = cn_printf(cn, "%d", cprm->cpu);
> > > > > break;
> > > > > + /* pidfd number */
> > > > > + case 'F': {
> > > > > + /*
> > > > > + * Installing a pidfd only makes sense if
> > > > > + * we actually spawn a usermode helper.
> > > > > + */
> > > > > + if (!ispipe)
> > > > > + break;
> > > > > +
> > > > > + /*
> > > > > + * Note that we'll install a pidfd for the
> > > > > + * thread-group leader. We know that task
> > > > > + * linkage hasn't been removed yet and even if
> > > > > + * this @current isn't the actual thread-group
> > > > > + * leader we know that the thread-group leader
> > > > > + * cannot be reaped until @current has exited.
> > > > > + */
> > > > > + cprm->pid = task_tgid(current);
> > > > > + err = cn_printf(cn, "%d", COREDUMP_PIDFD_NUMBER);
> > > > > + break;
> > > > > + }
> > > > > default:
> > > > > break;
> > > > > }
> > > > >
> > > >
> > > > I tried this change with Apport: I took the Ubuntu mainline kernel build
> > > > https://kernel.ubuntu.com/mainline/daily/2025-04-24/ (that refers to
> > > > mainline commit e54f9b0410347c49b7ffdd495578811e70d7a407) and applied
> > > > these three patches on top. Then I modified Apport to take the
> > > > additional `-F%F` and tested that on Ubuntu 25.04 (plucky). The result
> > > > is the coredump failed as long as there was `-F%F` on
> > >
> > > I have no clue what -F%F is and whether that leading -F is something
> > > specific to Apport but the specifier is %F not -F%F. For example:
> > >
> > > > cat /proc/sys/kernel/core_pattern
> > > |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h %F
> > >
> > > And note that this requires the pipe logic to be used, aka "|" needs to
> > > be specified. Without it this doesn't make sense.
> >
> > Apport takes short option parameters. They match the kernel template
> > specifiers. The failing pattern:
> >
> > $ cat /proc/sys/kernel/core_pattern
> > > /usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -F%F -- %E
> >
> > Once I drop %F Apport is called without issues:
> >
> > $ cat /proc/sys/kernel/core_pattern
> > > /usr/share/apport/apport -p%p -s%s -c%c -d%d -P%P -u%u -g%g -- %E
>
> Youm must have CONFIG_DEBUG_VFS=y enabled where we trample the pidfs
> file we just allocated. It's a debug only assert. I've removed it now
> and pushed it to vfs-6.16.coredump. Can you either try with that or
> simply unset CONFIG_DEBUG_VFS and retest.
Yes, the Ubuntu mainline builds have CONFIG_DEBUG_VFS=y set. I tried
your vfs-6.16.coredump and it works. Thanks.
--
Benjamin Drung
Debian & Ubuntu Developer
On 04/14, Christian Brauner wrote:
>
> -static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
> +static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
> {
> struct file *files[2];
> struct coredump_params *cp = (struct coredump_params *)info->data;
> int err;
>
> + if (cp->pid) {
> + struct file *pidfs_file __free(fput) = NULL;
> +
> + pidfs_file = pidfs_alloc_file(cp->pid, 0);
> + if (IS_ERR(pidfs_file))
> + return PTR_ERR(pidfs_file);
> +
> + /*
> + * Usermode helpers are childen of either
> + * system_unbound_wq or of kthreadd. So we know that
> + * we're starting off with a clean file descriptor
> + * table. So we should always be able to use
> + * COREDUMP_PIDFD_NUMBER as our file descriptor value.
> + */
> + VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
> +
> + err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
> + if (err < 0)
> + return err;
Yes, but if replace_fd() succeeds we need to nullify pidfs_file
to avoid fput from __free(fput) ?
And I think in this case __free(fput) doesn't buy too much, but
up to you.
Oleg.
On 04/14, Oleg Nesterov wrote:
>
> On 04/14, Christian Brauner wrote:
> >
> > -static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
> > +static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
> > {
> > struct file *files[2];
> > struct coredump_params *cp = (struct coredump_params *)info->data;
> > int err;
> >
> > + if (cp->pid) {
> > + struct file *pidfs_file __free(fput) = NULL;
> > +
> > + pidfs_file = pidfs_alloc_file(cp->pid, 0);
> > + if (IS_ERR(pidfs_file))
> > + return PTR_ERR(pidfs_file);
> > +
> > + /*
> > + * Usermode helpers are childen of either
> > + * system_unbound_wq or of kthreadd. So we know that
> > + * we're starting off with a clean file descriptor
> > + * table. So we should always be able to use
> > + * COREDUMP_PIDFD_NUMBER as our file descriptor value.
> > + */
> > + VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
> > +
> > + err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
> > + if (err < 0)
> > + return err;
>
> Yes, but if replace_fd() succeeds we need to nullify pidfs_file
> to avoid fput from __free(fput) ?
Aah, please ignore me ;) replace_fd/do_dup2 does get_file() .
For this series:
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
On Mon, Apr 14, 2025 at 04:28:07PM +0200, Oleg Nesterov wrote:
> On 04/14, Oleg Nesterov wrote:
> >
> > On 04/14, Christian Brauner wrote:
> > >
> > > -static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
> > > +static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
> > > {
> > > struct file *files[2];
> > > struct coredump_params *cp = (struct coredump_params *)info->data;
> > > int err;
> > >
> > > + if (cp->pid) {
> > > + struct file *pidfs_file __free(fput) = NULL;
> > > +
> > > + pidfs_file = pidfs_alloc_file(cp->pid, 0);
> > > + if (IS_ERR(pidfs_file))
> > > + return PTR_ERR(pidfs_file);
> > > +
> > > + /*
> > > + * Usermode helpers are childen of either
> > > + * system_unbound_wq or of kthreadd. So we know that
> > > + * we're starting off with a clean file descriptor
> > > + * table. So we should always be able to use
> > > + * COREDUMP_PIDFD_NUMBER as our file descriptor value.
> > > + */
> > > + VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
> > > +
> > > + err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
> > > + if (err < 0)
> > > + return err;
> >
> > Yes, but if replace_fd() succeeds we need to nullify pidfs_file
> > to avoid fput from __free(fput) ?
>
> Aah, please ignore me ;) replace_fd/do_dup2 does get_file() .
>
> For this series:
Thanks for the excellent review as usual!
On Mon, Apr 14, 2025 at 04:14:50PM +0200, Oleg Nesterov wrote:
> On 04/14, Christian Brauner wrote:
> >
> > -static int umh_pipe_setup(struct subprocess_info *info, struct cred *new)
> > +static int umh_coredump_setup(struct subprocess_info *info, struct cred *new)
> > {
> > struct file *files[2];
> > struct coredump_params *cp = (struct coredump_params *)info->data;
> > int err;
> >
> > + if (cp->pid) {
> > + struct file *pidfs_file __free(fput) = NULL;
> > +
> > + pidfs_file = pidfs_alloc_file(cp->pid, 0);
> > + if (IS_ERR(pidfs_file))
> > + return PTR_ERR(pidfs_file);
> > +
> > + /*
> > + * Usermode helpers are childen of either
> > + * system_unbound_wq or of kthreadd. So we know that
> > + * we're starting off with a clean file descriptor
> > + * table. So we should always be able to use
> > + * COREDUMP_PIDFD_NUMBER as our file descriptor value.
> > + */
> > + VFS_WARN_ON_ONCE((pidfs_file = fget_raw(COREDUMP_PIDFD_NUMBER)) != NULL);
> > +
> > + err = replace_fd(COREDUMP_PIDFD_NUMBER, pidfs_file, 0);
> > + if (err < 0)
> > + return err;
>
> Yes, but if replace_fd() succeeds we need to nullify pidfs_file
> to avoid fput from __free(fput) ?
No, since replace_fd() takes its own reference via do_dup2():
replace_fd()
-> do_dup2()
{
get_file(file)
rcu_assign_pointer(fdt->fd[fd], file);
}
so we always need to call it. I had a comment about this in the previous
patchset so people don't get confused. I can add it back.
Let me know if you're happy with this otherwise.
>
> And I think in this case __free(fput) doesn't buy too much, but
> up to you.
>
> Oleg.
>
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