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SubjectRe: [PATCH] selftests/x86: Support Atom for syscall_arg_fault test
On Thu, Mar 7, 2019 at 12:22 AM Tong Bo <bo.tong@intel.com> wrote:
>
> Atom-based CPUs trigger stack fault when invoke 32-bit SYSENTER instruction
> with invalid register values. So we also need sigbus handling in this case.
>
> Following is assembly when the fault expception happens.
>
> (gdb) disassemble $eip
> Dump of assembler code for function __kernel_vsyscall:
> 0xf7fd8fe0 <+0>: push %ecx
> 0xf7fd8fe1 <+1>: push %edx
> 0xf7fd8fe2 <+2>: push %ebp
> 0xf7fd8fe3 <+3>: mov %esp,%ebp
> 0xf7fd8fe5 <+5>: sysenter
> 0xf7fd8fe7 <+7>: int $0x80
> => 0xf7fd8fe9 <+9>: pop %ebp
> 0xf7fd8fea <+10>: pop %edx
> 0xf7fd8feb <+11>: pop %ecx
> 0xf7fd8fec <+12>: ret
> End of assembler dump.
>
> Accroding to Intel SDM, this could also be a Stack Segment Fault(#SS, 12),
> except a normal Page Fault(#PF, 14).

Really? What is in the SS register that makes a stack segment fault
possible? Is it because we're overflowing ESP? Would a value like -5
instead of -1 in the register change things?

Anyway, I'm okay with the patch, but I think that you should improve
the comment:

> - sethandler(SIGSEGV, sigsegv, SA_ONSTACK);
> + sethandler(SIGSEGV, sigsegv_or_sigbus, SA_ONSTACK);
> + /* Atom CPUs may trigger sigbus for below SYSENTER exception case */
> + sethandler(SIGBUS, sigsegv_or_sigbus, SA_ONSTACK);

How about "The actual exception can vary. On Atom CPUs, we get #SS
instead of #PF when the vDSO fails to access the stack when ESP is too
close to 2^32, and #SS causes SIGBUS".

--Andy

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