As userspace on UML/!MMU also need to configure %fs register when it is
running to correctly access thread structure, host syscalls implemented
in os-Linux drivers may be puzzled when they are called. Thus it has to
configure %fs register via arch_prctl(SET_FS) on every host syscalls.
Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
---
arch/um/include/shared/os.h | 6 +++
arch/um/os-Linux/process.c | 6 +++
arch/um/os-Linux/start_up.c | 21 +++++++++
arch/x86/um/nommu/do_syscall_64.c | 37 ++++++++++++++++
arch/x86/um/nommu/syscalls_64.c | 71 +++++++++++++++++++++++++++++++
5 files changed, 141 insertions(+)
diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index 1251f08e26d0..7c6a8bc0447c 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -189,6 +189,7 @@ extern void check_host_supports_tls(int *supports_tls, int *tls_min);
extern void get_host_cpu_features(
void (*flags_helper_func)(char *line),
void (*cache_helper_func)(char *line));
+extern int host_has_fsgsbase;
/* mem.c */
extern int create_mem_file(unsigned long long len);
@@ -213,6 +214,11 @@ extern int os_protect_memory(void *addr, unsigned long len,
extern int os_unmap_memory(void *addr, int len);
extern int os_drop_memory(void *addr, int length);
extern int can_drop_memory(void);
+extern int os_arch_prctl(int pid, int option, unsigned long *arg);
+#ifndef CONFIG_MMU
+extern long long host_fs;
+#endif
+
void os_set_pdeathsig(void);
diff --git a/arch/um/os-Linux/process.c b/arch/um/os-Linux/process.c
index 8a1ab59a089f..3a6d34ccd12b 100644
--- a/arch/um/os-Linux/process.c
+++ b/arch/um/os-Linux/process.c
@@ -16,6 +16,7 @@
#include <sys/prctl.h>
#include <sys/wait.h>
#include <asm/unistd.h>
+#include <sys/syscall.h> /* For SYS_xxx definitions */
#include <linux/threads.h>
#include <init.h>
#include <longjmp.h>
@@ -178,6 +179,11 @@ int __init can_drop_memory(void)
return ok;
}
+int os_arch_prctl(int pid, int option, unsigned long *arg2)
+{
+ return syscall(SYS_arch_prctl, option, arg2);
+}
+
void init_new_thread_signals(void)
{
set_handler(SIGSEGV);
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index 4e1f05360c49..55dd92bd2a0b 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -20,6 +20,8 @@
#include <sys/resource.h>
#include <asm/ldt.h>
#include <asm/unistd.h>
+#include <sys/auxv.h>
+#include <asm/hwcap2.h>
#include <init.h>
#include <os.h>
#include <kern_util.h>
@@ -36,6 +38,8 @@
#include <skas.h>
#include "internal.h"
+int host_has_fsgsbase;
+
static void ptrace_child(void)
{
int ret;
@@ -459,6 +463,20 @@ __uml_setup("seccomp=", uml_seccomp_config,
" This is insecure and should only be used with a trusted userspace\n\n"
);
+static void __init check_fsgsbase(void)
+{
+ unsigned long auxv = getauxval(AT_HWCAP2);
+
+ os_info("Checking FSGSBASE instructions...");
+ if (auxv & HWCAP2_FSGSBASE) {
+ host_has_fsgsbase = 1;
+ os_info("OK\n");
+ } else {
+ host_has_fsgsbase = 0;
+ os_info("disabled\n");
+ }
+}
+
void __init os_early_checks(void)
{
int pid;
@@ -484,6 +502,9 @@ void __init os_early_checks(void)
using_seccomp = 0;
check_ptrace();
+ /* probe fsgsbase instruction */
+ check_fsgsbase();
+
pid = start_ptraced_child();
if (init_pid_registers(pid))
fatal("Failed to initialize default registers");
diff --git a/arch/x86/um/nommu/do_syscall_64.c b/arch/x86/um/nommu/do_syscall_64.c
index 5d0fa83e7fdc..796beb0089fc 100644
--- a/arch/x86/um/nommu/do_syscall_64.c
+++ b/arch/x86/um/nommu/do_syscall_64.c
@@ -2,10 +2,38 @@
#include <linux/kernel.h>
#include <linux/ptrace.h>
+#include <asm/fsgsbase.h>
+#include <asm/prctl.h>
#include <kern_util.h>
#include <sysdep/syscalls.h>
#include <os.h>
+static int os_x86_arch_prctl(int pid, int option, unsigned long *arg2)
+{
+ if (!host_has_fsgsbase)
+ return os_arch_prctl(pid, option, arg2);
+
+ switch (option) {
+ case ARCH_SET_FS:
+ wrfsbase(*arg2);
+ break;
+ case ARCH_SET_GS:
+ wrgsbase(*arg2);
+ break;
+ case ARCH_GET_FS:
+ *arg2 = rdfsbase();
+ break;
+ case ARCH_GET_GS:
+ *arg2 = rdgsbase();
+ break;
+ default:
+ pr_warn("%s: unsupported option: 0x%x", __func__, option);
+ break;
+ }
+
+ return 0;
+}
+
__visible void do_syscall_64(struct pt_regs *regs)
{
int syscall;
@@ -17,6 +45,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
syscall, (unsigned long)current,
(unsigned long)sys_call_table[syscall]);
+ /* set fs register to the original host one */
+ os_x86_arch_prctl(0, ARCH_SET_FS, (void *)host_fs);
+
if (likely(syscall < NR_syscalls)) {
PT_REGS_SET_SYSCALL_RETURN(regs,
EXECUTE_SYSCALL(syscall, regs));
@@ -34,4 +65,10 @@ __visible void do_syscall_64(struct pt_regs *regs)
/* force do_signal() --> is_syscall() */
set_thread_flag(TIF_SIGPENDING);
interrupt_end();
+
+ /* restore back fs register to userspace configured one */
+ os_x86_arch_prctl(0, ARCH_SET_FS,
+ (void *)(current->thread.regs.regs.gp[FS_BASE
+ / sizeof(unsigned long)]));
+
}
diff --git a/arch/x86/um/nommu/syscalls_64.c b/arch/x86/um/nommu/syscalls_64.c
index c78c442aed1d..5bb6d55b4bb5 100644
--- a/arch/x86/um/nommu/syscalls_64.c
+++ b/arch/x86/um/nommu/syscalls_64.c
@@ -13,8 +13,70 @@
#include <asm/prctl.h> /* XXX This should get the constants from libc */
#include <registers.h>
#include <os.h>
+#include <asm/thread_info.h>
+#include <asm/mman.h>
#include "syscalls.h"
+/*
+ * The guest libc can change FS, which confuses the host libc.
+ * In fact, changing FS directly is not supported (check
+ * man arch_prctl). So, whenever we make a host syscall,
+ * we should be changing FS to the original FS (not the
+ * one set by the guest libc). This original FS is stored
+ * in host_fs.
+ */
+long long host_fs = -1;
+
+long arch_prctl(struct task_struct *task, int option,
+ unsigned long __user *arg2)
+{
+ long ret = -EINVAL;
+ unsigned long *ptr = arg2, tmp;
+
+ switch (option) {
+ case ARCH_SET_FS:
+ if (host_fs == -1)
+ os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
+ ret = 0;
+ break;
+ case ARCH_SET_GS:
+ ret = 0;
+ break;
+ case ARCH_GET_FS:
+ case ARCH_GET_GS:
+ ptr = &tmp;
+ break;
+ }
+
+ ret = os_arch_prctl(0, option, ptr);
+ if (ret)
+ return ret;
+
+ switch (option) {
+ case ARCH_SET_FS:
+ current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)] =
+ (unsigned long) arg2;
+ break;
+ case ARCH_SET_GS:
+ current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)] =
+ (unsigned long) arg2;
+ break;
+ case ARCH_GET_FS:
+ ret = put_user(current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)], arg2);
+ break;
+ case ARCH_GET_GS:
+ ret = put_user(current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)], arg2);
+ break;
+ }
+
+ return ret;
+}
+
+SYSCALL_DEFINE2(arch_prctl, int, option, unsigned long, arg2)
+{
+ return arch_prctl(current, option, (unsigned long __user *) arg2);
+}
+
void arch_switch_to(struct task_struct *to)
{
/*
@@ -42,3 +104,12 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
}
+
+static int __init um_nommu_setup_hostfs(void)
+{
+ /* initialize the host_fs value at boottime */
+ os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
+
+ return 0;
+}
+arch_initcall(um_nommu_setup_hostfs);
--
2.43.0On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> As userspace on UML/!MMU also need to configure %fs register when it is
> running to correctly access thread structure, host syscalls implemented
> in os-Linux drivers may be puzzled when they are called. Thus it has to
> configure %fs register via arch_prctl(SET_FS) on every host syscalls.
Really, I still think that we should "just" get rid of libc entirely
inside UML. That would avoid so many weird/potential issues …
Doesn't change the fact that FS/GS needs to be restored when doing
thread switches and such. Though one might be able to do it entirely
within arch_switch_to then.
Benjamin
>
> Signed-off-by: Hajime Tazaki <thehajime@gmail.com>
> Signed-off-by: Ricardo Koller <ricarkol@google.com>
> ---
> arch/um/include/shared/os.h | 6 +++
> arch/um/os-Linux/process.c | 6 +++
> arch/um/os-Linux/start_up.c | 21 +++++++++
> arch/x86/um/nommu/do_syscall_64.c | 37 ++++++++++++++++
> arch/x86/um/nommu/syscalls_64.c | 71 +++++++++++++++++++++++++++++++
> 5 files changed, 141 insertions(+)
>
> diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
> index 1251f08e26d0..7c6a8bc0447c 100644
> --- a/arch/um/include/shared/os.h
> +++ b/arch/um/include/shared/os.h
> @@ -189,6 +189,7 @@ extern void check_host_supports_tls(int *supports_tls, int *tls_min);
> extern void get_host_cpu_features(
> void (*flags_helper_func)(char *line),
> void (*cache_helper_func)(char *line));
> +extern int host_has_fsgsbase;
>
> /* mem.c */
> extern int create_mem_file(unsigned long long len);
> @@ -213,6 +214,11 @@ extern int os_protect_memory(void *addr, unsigned long len,
> extern int os_unmap_memory(void *addr, int len);
> extern int os_drop_memory(void *addr, int length);
> extern int can_drop_memory(void);
> +extern int os_arch_prctl(int pid, int option, unsigned long *arg);
> +#ifndef CONFIG_MMU
> +extern long long host_fs;
> +#endif
> +
>
> void os_set_pdeathsig(void);
>
> diff --git a/arch/um/os-Linux/process.c b/arch/um/os-Linux/process.c
> index 8a1ab59a089f..3a6d34ccd12b 100644
> --- a/arch/um/os-Linux/process.c
> +++ b/arch/um/os-Linux/process.c
> @@ -16,6 +16,7 @@
> #include <sys/prctl.h>
> #include <sys/wait.h>
> #include <asm/unistd.h>
> +#include <sys/syscall.h> /* For SYS_xxx definitions */
> #include <linux/threads.h>
> #include <init.h>
> #include <longjmp.h>
> @@ -178,6 +179,11 @@ int __init can_drop_memory(void)
> return ok;
> }
>
> +int os_arch_prctl(int pid, int option, unsigned long *arg2)
> +{
> + return syscall(SYS_arch_prctl, option, arg2);
> +}
> +
> void init_new_thread_signals(void)
> {
> set_handler(SIGSEGV);
> diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
> index 4e1f05360c49..55dd92bd2a0b 100644
> --- a/arch/um/os-Linux/start_up.c
> +++ b/arch/um/os-Linux/start_up.c
> @@ -20,6 +20,8 @@
> #include <sys/resource.h>
> #include <asm/ldt.h>
> #include <asm/unistd.h>
> +#include <sys/auxv.h>
> +#include <asm/hwcap2.h>
> #include <init.h>
> #include <os.h>
> #include <kern_util.h>
> @@ -36,6 +38,8 @@
> #include <skas.h>
> #include "internal.h"
>
> +int host_has_fsgsbase;
> +
> static void ptrace_child(void)
> {
> int ret;
> @@ -459,6 +463,20 @@ __uml_setup("seccomp=", uml_seccomp_config,
> " This is insecure and should only be used with a trusted userspace\n\n"
> );
>
> +static void __init check_fsgsbase(void)
> +{
> + unsigned long auxv = getauxval(AT_HWCAP2);
> +
> + os_info("Checking FSGSBASE instructions...");
> + if (auxv & HWCAP2_FSGSBASE) {
> + host_has_fsgsbase = 1;
> + os_info("OK\n");
> + } else {
> + host_has_fsgsbase = 0;
> + os_info("disabled\n");
> + }
> +}
> +
> void __init os_early_checks(void)
> {
> int pid;
> @@ -484,6 +502,9 @@ void __init os_early_checks(void)
> using_seccomp = 0;
> check_ptrace();
>
> + /* probe fsgsbase instruction */
> + check_fsgsbase();
> +
> pid = start_ptraced_child();
> if (init_pid_registers(pid))
> fatal("Failed to initialize default registers");
> diff --git a/arch/x86/um/nommu/do_syscall_64.c b/arch/x86/um/nommu/do_syscall_64.c
> index 5d0fa83e7fdc..796beb0089fc 100644
> --- a/arch/x86/um/nommu/do_syscall_64.c
> +++ b/arch/x86/um/nommu/do_syscall_64.c
> @@ -2,10 +2,38 @@
>
> #include <linux/kernel.h>
> #include <linux/ptrace.h>
> +#include <asm/fsgsbase.h>
> +#include <asm/prctl.h>
> #include <kern_util.h>
> #include <sysdep/syscalls.h>
> #include <os.h>
>
> +static int os_x86_arch_prctl(int pid, int option, unsigned long *arg2)
> +{
> + if (!host_has_fsgsbase)
> + return os_arch_prctl(pid, option, arg2);
> +
> + switch (option) {
> + case ARCH_SET_FS:
> + wrfsbase(*arg2);
> + break;
> + case ARCH_SET_GS:
> + wrgsbase(*arg2);
> + break;
> + case ARCH_GET_FS:
> + *arg2 = rdfsbase();
> + break;
> + case ARCH_GET_GS:
> + *arg2 = rdgsbase();
> + break;
> + default:
> + pr_warn("%s: unsupported option: 0x%x", __func__, option);
> + break;
> + }
> +
> + return 0;
> +}
> +
> __visible void do_syscall_64(struct pt_regs *regs)
> {
> int syscall;
> @@ -17,6 +45,9 @@ __visible void do_syscall_64(struct pt_regs *regs)
> syscall, (unsigned long)current,
> (unsigned long)sys_call_table[syscall]);
>
> + /* set fs register to the original host one */
> + os_x86_arch_prctl(0, ARCH_SET_FS, (void *)host_fs);
> +
> if (likely(syscall < NR_syscalls)) {
> PT_REGS_SET_SYSCALL_RETURN(regs,
> EXECUTE_SYSCALL(syscall, regs));
> @@ -34,4 +65,10 @@ __visible void do_syscall_64(struct pt_regs *regs)
> /* force do_signal() --> is_syscall() */
> set_thread_flag(TIF_SIGPENDING);
> interrupt_end();
> +
> + /* restore back fs register to userspace configured one */
> + os_x86_arch_prctl(0, ARCH_SET_FS,
> + (void *)(current->thread.regs.regs.gp[FS_BASE
> + / sizeof(unsigned long)]));
> +
> }
> diff --git a/arch/x86/um/nommu/syscalls_64.c b/arch/x86/um/nommu/syscalls_64.c
> index c78c442aed1d..5bb6d55b4bb5 100644
> --- a/arch/x86/um/nommu/syscalls_64.c
> +++ b/arch/x86/um/nommu/syscalls_64.c
> @@ -13,8 +13,70 @@
> #include <asm/prctl.h> /* XXX This should get the constants from libc */
> #include <registers.h>
> #include <os.h>
> +#include <asm/thread_info.h>
> +#include <asm/mman.h>
> #include "syscalls.h"
>
> +/*
> + * The guest libc can change FS, which confuses the host libc.
> + * In fact, changing FS directly is not supported (check
> + * man arch_prctl). So, whenever we make a host syscall,
> + * we should be changing FS to the original FS (not the
> + * one set by the guest libc). This original FS is stored
> + * in host_fs.
> + */
> +long long host_fs = -1;
> +
> +long arch_prctl(struct task_struct *task, int option,
> + unsigned long __user *arg2)
> +{
> + long ret = -EINVAL;
> + unsigned long *ptr = arg2, tmp;
> +
> + switch (option) {
> + case ARCH_SET_FS:
> + if (host_fs == -1)
> + os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
> + ret = 0;
> + break;
> + case ARCH_SET_GS:
> + ret = 0;
> + break;
> + case ARCH_GET_FS:
> + case ARCH_GET_GS:
> + ptr = &tmp;
> + break;
> + }
> +
> + ret = os_arch_prctl(0, option, ptr);
> + if (ret)
> + return ret;
> +
> + switch (option) {
> + case ARCH_SET_FS:
> + current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)] =
> + (unsigned long) arg2;
> + break;
> + case ARCH_SET_GS:
> + current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)] =
> + (unsigned long) arg2;
> + break;
> + case ARCH_GET_FS:
> + ret = put_user(current->thread.regs.regs.gp[FS_BASE / sizeof(unsigned long)], arg2);
> + break;
> + case ARCH_GET_GS:
> + ret = put_user(current->thread.regs.regs.gp[GS_BASE / sizeof(unsigned long)], arg2);
> + break;
> + }
> +
> + return ret;
> +}
> +
> +SYSCALL_DEFINE2(arch_prctl, int, option, unsigned long, arg2)
> +{
> + return arch_prctl(current, option, (unsigned long __user *) arg2);
> +}
> +
> void arch_switch_to(struct task_struct *to)
> {
> /*
> @@ -42,3 +104,12 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
>
> return ksys_mmap_pgoff(addr, len, prot, flags, fd, off >> PAGE_SHIFT);
> }
> +
> +static int __init um_nommu_setup_hostfs(void)
> +{
> + /* initialize the host_fs value at boottime */
> + os_arch_prctl(0, ARCH_GET_FS, (void *)&host_fs);
> +
> + return 0;
> +}
> +arch_initcall(um_nommu_setup_hostfs);
On Thu, 19 Jun 2025 19:40:49 +0900,
Benjamin Berg wrote:
>
> On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> > As userspace on UML/!MMU also need to configure %fs register when it is
> > running to correctly access thread structure, host syscalls implemented
> > in os-Linux drivers may be puzzled when they are called. Thus it has to
> > configure %fs register via arch_prctl(SET_FS) on every host syscalls.
>
> Really, I still think that we should "just" get rid of libc entirely
> inside UML. That would avoid so many weird/potential issues …
I'm not sure if I understand your point.
Q1) what do you mean by 'get rid of libc entirely' here ?
do you mean the following code block adds the dependency ?
+ int os_arch_prctl(int pid, int option, unsigned long *arg2)
+ {
+ return syscall(SYS_arch_prctl, option, arg2);
+ }
I guess this can be replaced with inline assembly instead of using
libc's one. but this is the code under os-Linux, which I thought we're
allowed to use the host code ?
Q2) "That would avoid so many weird/potential issues …"
I'm new to this; I'm wondering what kind of issues did you see ?
> Doesn't change the fact that FS/GS needs to be restored when doing
> thread switches and such. Though one might be able to do it entirely
> within arch_switch_to then.
I believe this is already done in arch_switch_to. This particular
patch does the control to the host context.
-- Hajime
On Thu, 2025-06-19 at 21:22 +0900, Hajime Tazaki wrote:
>
> On Thu, 19 Jun 2025 19:40:49 +0900,
> Benjamin Berg wrote:
> >
> > On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> > > As userspace on UML/!MMU also need to configure %fs register when
> > > it is
> > > running to correctly access thread structure, host syscalls
> > > implemented
> > > in os-Linux drivers may be puzzled when they are called. Thus it
> > > has to
> > > configure %fs register via arch_prctl(SET_FS) on every host
> > > syscalls.
> >
> > Really, I still think that we should "just" get rid of libc
> > entirely
> > inside UML. That would avoid so many weird/potential issues …
>
> I'm not sure if I understand your point.
>
> Q1) what do you mean by 'get rid of libc entirely' here ?
> do you mean the following code block adds the dependency ?
> + int os_arch_prctl(int pid, int option, unsigned long *arg2)
> + {
> + return syscall(SYS_arch_prctl, option, arg2);
> + }
>
> I guess this can be replaced with inline assembly instead of using
> libc's one. but this is the code under os-Linux, which I thought
> we're
> allowed to use the host code ?
>
> Q2) "That would avoid so many weird/potential issues …"
> I'm new to this; I'm wondering what kind of issues did you see ?
Oh, I am just being annoyed by libc in general in UM. It isn't specific
to this patchset.
An example is that we need to keep malloc() working for libc. Which I
would think is kind of weird. Or we had issues because libc turned on
rseq and that was inherited into userspace, causing random crashes and
such.
> > Doesn't change the fact that FS/GS needs to be restored when doing
> > thread switches and such. Though one might be able to do it
> > entirely
> > within arch_switch_to then.
>
> I believe this is already done in arch_switch_to. This particular
> patch does the control to the host context.
OK, need to look at that again a bit. I haven't really wrapped my mind
around how everything fits together, so I probably got some stuff
wrong.
Benjamin
On Thu, 19 Jun 2025 21:38:47 +0900,
Benjamin Berg wrote:
>
> On Thu, 2025-06-19 at 21:22 +0900, Hajime Tazaki wrote:
> >
> > On Thu, 19 Jun 2025 19:40:49 +0900,
> > Benjamin Berg wrote:
> > >
> > > On Thu, 2025-06-19 at 10:04 +0900, Hajime Tazaki wrote:
> > > > As userspace on UML/!MMU also need to configure %fs register when
> > > > it is
> > > > running to correctly access thread structure, host syscalls
> > > > implemented
> > > > in os-Linux drivers may be puzzled when they are called. Thus it
> > > > has to
> > > > configure %fs register via arch_prctl(SET_FS) on every host
> > > > syscalls.
> > >
> > > Really, I still think that we should "just" get rid of libc
> > > entirely
> > > inside UML. That would avoid so many weird/potential issues …
> >
> > I'm not sure if I understand your point.
> >
> > Q1) what do you mean by 'get rid of libc entirely' here ?
> > do you mean the following code block adds the dependency ?
> > + int os_arch_prctl(int pid, int option, unsigned long *arg2)
> > + {
> > + return syscall(SYS_arch_prctl, option, arg2);
> > + }
> >
> > I guess this can be replaced with inline assembly instead of using
> > libc's one. but this is the code under os-Linux, which I thought
> > we're
> > allowed to use the host code ?
> >
> > Q2) "That would avoid so many weird/potential issues …"
> > I'm new to this; I'm wondering what kind of issues did you see ?
>
> Oh, I am just being annoyed by libc in general in UM. It isn't specific
> to this patchset.
>
> An example is that we need to keep malloc() working for libc. Which I
> would think is kind of weird. Or we had issues because libc turned on
> rseq and that was inherited into userspace, causing random crashes and
> such.
I understand, thanks for the input.
> > > Doesn't change the fact that FS/GS needs to be restored when doing
> > > thread switches and such. Though one might be able to do it
> > > entirely
> > > within arch_switch_to then.
> >
> > I believe this is already done in arch_switch_to. This particular
> > patch does the control to the host context.
>
> OK, need to look at that again a bit. I haven't really wrapped my mind
> around how everything fits together, so I probably got some stuff
> wrong.
anyway, thanks for your time to look at this.
-- Hajime
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