Re: [PATCH v2 1/3] powerpc: Don't force ENOSYS as error on syscall fail

From: Michael Ellerman
Date: Thu Feb 12 2015 - 00:24:56 EST

On Wed, 2015-02-11 at 08:36 +0000, Bogdan Purcareata wrote:
> In certain scenarios - e.g. seccomp filtering with ERRNO as default action -
> the system call fails for other reasons than the syscall not being available.
> The seccomp filter can be configured to store a user-defined error code on
> return from a blacklisted syscall. Don't always set ENOSYS on
> do_syscall_trace_enter failure.
> v2:
> - move setting ENOSYS as errno from the syscall entry assembly to
> do_syscall_trace_enter, only in the specific case

> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 194e46d..0111e04 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -269,7 +269,6 @@ syscall_dotrace:
> b .Lsyscall_dotrace_cont
> syscall_enosys:
> - li r3,-ENOSYS
> b syscall_exit

This still looks wrong to me.

On 64 bit we do:

ld r10,TI_FLAGS(r11)
andi. r11,r10,_TIF_SYSCALL_DOTRACE
bne syscall_dotrace
cmpldi 0,r0,NR_syscalls
bge- syscall_enosys

li r3,-ENOSYS
b .Lsyscall_exit

Your patch removes the load of ENOSYS.

Which means if we're not doing syscall tracing, and we get an out-of-bounds
syscall number, we'll return with something random on r3. Won't we?

The 32-bit code looks more or less similar, although the label has a different


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