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

[Denys Vlasenko]

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.

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 */
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