Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs"

[Sabrina Dubroca]

2015-02-26, 14:54:33 +0100, Denys Vlasenko wrote:
> On Thu, Feb 26, 2015 at 1:11 PM, Denys Vlasenko
> <vda.linux@xxxxxxxxxxxxxx> wrote:
> > On Thu, Feb 26, 2015 at 10:55 AM, Denys Vlasenko
> > <vda.linux@xxxxxxxxxxxxxx> wrote:
> >> On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >> In addition to my previous tests, I ran my home machine with
> >> patched kernel. Unfortunately, it works for me :(
> >>
> >> Will try on yet another machine.
> >
> > And voila, it does happen on another machine :)
> >
> > I'm debugging it right now. Looks like 64-bit syscalls just stop working
> > at some point in new processes. That is, existing process is alive and well,
> > but children get SEGV after fork (most likely on any syscall64 they do,
> > not after fork per se. They eventually manage to kill themselves -
> > not trivial when exit syscall isn't working either - by tripping on HLT insn).
> >
> > 32-bit syscalls (int 80) continue to work. Fork, exec, whatever you want.
> > I have static 32-bit busybox binary and everything works there.
> >
> > Also, any 64-bit process which was under strace continues to work correctly,
> > including forks and execs.
> >
> > This points towards some bug on fast path sysret64 code. Looking for it.
> 
> audit=0 makes crashes disappear.

Ah, yes.

> I found the problem. If syscall_trace_enter_phase1 returns 0,
> I restore %rax from pt_regs->ax, but should restore it from
> pt_regs->orig_ax:
> 
>         call syscall_trace_enter_phase1
>         test %rax, %rax
>         jnz tracesys_phase2             /* if needed, run the slow path */
> -       RESTORE_C_REGS                  /* else restore clobbered regs */
> +       RESTORE_C_REGS_EXCEPT_RAX       /* else restore clobbered regs */
> +       movq ORIG_RAX-ARGOFFSET(%rsp),%rax
>         jmp system_call_fastpath        /*      and return to the fast path */

with s/-ARGOFFSET// on top of next-20150224, that works.

Thanks, Denys.

-- 
Sabrina
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