For certain paths in /proc, the open syscall is intercepted and the
returned file descriptor points to a temporary file with emulated
contents.
If TMPDIR is not accessible or writable for the current user (for
example in a read-only mounted chroot or container) tools such as ps
from procps may fail unexpectedly. Trying to read one of these paths
such as /proc/self/stat would return an error such as ENOENT or EROFS.
To relax the requirement on a writable TMPDIR, use memfd_create()
instead to create an anonymous file and return its file descriptor.
Signed-off-by: Rainer Müller <raimue@codingfarm.de>
---
v2: no more #ifdefs, use stub from util/memfd.c with ENOSYS fallback,
tested with 'strace -e fault=memfd_create'
---
linux-user/syscall.c | 22 ++++++++++++++--------
1 file changed, 14 insertions(+), 8 deletions(-)
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 991b85e6b4..7b55726f25 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -8269,16 +8269,22 @@ static int do_openat(CPUArchState *cpu_env, int dirfd, const char *pathname, int
char filename[PATH_MAX];
int fd, r;
- /* create temporary file to map stat to */
- tmpdir = getenv("TMPDIR");
- if (!tmpdir)
- tmpdir = "/tmp";
- snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir);
- fd = mkstemp(filename);
+ fd = memfd_create("qemu-open", 0);
if (fd < 0) {
- return fd;
+ if (errno != ENOSYS) {
+ return fd;
+ }
+ /* create temporary file to map stat to */
+ tmpdir = getenv("TMPDIR");
+ if (!tmpdir)
+ tmpdir = "/tmp";
+ snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir);
+ fd = mkstemp(filename);
+ if (fd < 0) {
+ return fd;
+ }
+ unlink(filename);
}
- unlink(filename);
if ((r = fake_open->fill(cpu_env, fd))) {
int e = errno;
--
2.25.1
On 7/29/22 08:49, Rainer Müller wrote: > + /* create temporary file to map stat to */ > + tmpdir = getenv("TMPDIR"); > + if (!tmpdir) > + tmpdir = "/tmp"; > + snprintf(filename, sizeof(filename), "%s/qemu-open.XXXXXX", tmpdir); > + fd = mkstemp(filename); > + if (fd < 0) { > + return fd; > + } We've been using g_file_open_tmp elsewhere; probably good to follow suit here. r~
On 29/07/2022 18.01, Richard Henderson wrote: > On 7/29/22 08:49, Rainer Müller wrote: >> + /* create temporary file to map stat to */ >> + tmpdir = getenv("TMPDIR"); >> + if (!tmpdir) >> + tmpdir = "/tmp"; >> + snprintf(filename, sizeof(filename), >> "%s/qemu-open.XXXXXX", tmpdir); >> + fd = mkstemp(filename); >> + if (fd < 0) { >> + return fd; >> + } > > We've been using g_file_open_tmp elsewhere; probably good to follow suit > here. That seemed reasonable at first, but with regards to error handling it gets a bit complicated. The suggested g_file_open_tmp() would leave us with a GError only, but to return something meaningful to the caller we must set errno in this context. As far as I can see, there is no way to convert back to an errno from GError. With g_file_open_tmp() we could always set the same generic errno, but that would hide the real cause completely. I debugged this problem with this message that was confusing, but at least it gave away a hint: cat: can't open '/proc/self/stat': Read-only file system The other option would be to g_assert_true(fd >= 0) and kill the process in case opening the temporary file failed. This also feels wrong, as the caller could still recover from this state and continue. Rainer
On 7/29/22 14:19, Rainer Müller wrote: > On 29/07/2022 18.01, Richard Henderson wrote: >> On 7/29/22 08:49, Rainer Müller wrote: >>> + /* create temporary file to map stat to */ >>> + tmpdir = getenv("TMPDIR"); >>> + if (!tmpdir) >>> + tmpdir = "/tmp"; >>> + snprintf(filename, sizeof(filename), >>> "%s/qemu-open.XXXXXX", tmpdir); >>> + fd = mkstemp(filename); >>> + if (fd < 0) { >>> + return fd; >>> + } >> >> We've been using g_file_open_tmp elsewhere; probably good to follow suit >> here. > > That seemed reasonable at first, but with regards to error handling it > gets a bit complicated. > > The suggested g_file_open_tmp() would leave us with a GError only, but > to return something meaningful to the caller we must set errno in this > context. As far as I can see, there is no way to convert back to an > errno from GError. > > With g_file_open_tmp() we could always set the same generic errno, but > that would hide the real cause completely. I debugged this problem with > this message that was confusing, but at least it gave away a hint: > cat: can't open '/proc/self/stat': Read-only file system That's a good reason. Let's leave this alone then. Reviewed-by: Richard Henderson <richard.henderson@linaro.org> r~
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