Messages in this thread | | | From | Denys Vlasenko <> | Date | Thu, 26 Feb 2015 14:54:33 +0100 | Subject | Re: [PATCH 2/3 v3] x86: entry_64.S: always allocate complete "struct pt_regs" |
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On Thu, Feb 26, 2015 at 1:11 PM, Denys Vlasenko <vda.linux@googlemail.com> wrote: > On Thu, Feb 26, 2015 at 10:55 AM, Denys Vlasenko > <vda.linux@googlemail.com> wrote: >> On Wed, Feb 25, 2015 at 10:59 PM, Andy Lutomirski <luto@amacapital.net> 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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