Re: [RFC 3/3] x86/ptrace: down with test_thread_flag(TIF_IA32)
From: Dmitry Safonov
Date: Mon Apr 25 2016 - 14:10:36 EST
On 04/25/2016 08:14 PM, Dmitry Safonov wrote:
On 04/25/2016 07:53 PM, Andy Lutomirski wrote:
On Mon, Apr 25, 2016 at 9:12 AM, Dmitry Safonov
<dsafonov@xxxxxxxxxxxxx> wrote:
As the task isn't executing at the moment of {GET,SET}REGS,
return regset that corresponds to code selector.
So, for i386 elf binary that changed it's CS to __USER_CS
it will return full x86_64 register set.
That will change ABI: i.e, strace uses returned register size
to determine, in which mode the application is.
With the current ABI that way is buggy:
Oleg, any comment here?
int main(int argc, char **argv, char **envp)
{
printf("Here we exit\n");
fflush(stdout);
asm volatile ("int $0x80" : : "a" (__NR_exit), "D" (1));
printf("After exit\n");
return 0;
}
This program will confuse strace:
[tst]$ strace ./confuse 2>&1 | tail
brk(0x1ca1000) = 0x1ca1000
write(1, "Here we exit\n", 13Here we exit
) = 13
exit(1) = ?
<... exit resumed> strace: _exit returned!
) = ?
write(1, "After exit\n", 11After exit
) = 11
exit_group(0) = ?
+++ exited with 0 +++
So this ABI change should make PTRACE_GETREGSET more reliable and
this will be another step to drop TIF_{IA32,X32} flags.
Does strace start working again with this change? I suspect that
we'll eventually have to expose syscall_get_arch directly through
ptrace, but that's a project for another day.
Oh, crap, not yet - seems like, I failed with my test.
I'll resend this patch as will get it fixed, sorry.
I find out, what I have changed (and broke test):
> if (!user_64bit_mode(task_pt_regs(task)))
was
> if (task_thread_info(task)->status & TS_COMPAT)
That way the test runs now:
brk(NULL) = 0x1145000
brk(0x1167000) = 0x1167000
write(1, "Here we exit\n", 13Here we exit
) = 13
strace: [ Process PID=1608 runs in 32 bit mode. ]
umask(0) = 022
strace: [ Process PID=1608 runs in 64 bit mode. ]
write(1, "After exit\n", 11After exit
) = 11
exit_group(0) = ?
+++ exited with 0 +++
But I changed on signal patch rebase and now I'm
thinking: should it be
> if (task_thread_info(task)->status & TS_COMPAT ||
!user_64bit_mode(task_pt_regs(task)))
or what?
Should we count program that does compat syscall
as compatible even if it's in 64-bit mode?
I think this patch is fine, but I'm not a ptrace expert.
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
---
arch/x86/kernel/ptrace.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 0f4d2a5df2dc..d7d72f2f8b46 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -1387,7 +1387,7 @@ void update_regset_xstate_info(unsigned int
size, u64 xstate_mask)
const struct user_regset_view *task_user_regset_view(struct
task_struct *task)
{
#ifdef CONFIG_IA32_EMULATION
- if (test_tsk_thread_flag(task, TIF_IA32))
+ if (!user_64bit_mode(task_pt_regs(task)))
#endif
#if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
return &user_x86_32_view;
--
2.8.0
--
Regards,
Dmitry Safonov