Re: Syscall changes registers beyond %eax, on linux-i386

[george anzinger]

Mikael Pettersson wrote:
> 
> Daniel Jacobowitz writes:
>  > That's not going to help.  As Richard said, the memory in question
>  > belongs to the called function.  GCC knows this.  It can freely modify
>  > it.  The fact that the value of the parameter is const is a
>  > language-level, semantic thing.  It doesn't say anything about the
>  > const-ness of that memory.  Only the ABI does.
> 
> Does Linux/x86 even have a proper ABI document? I've never seen one.
> The closest I've seen would be the SVR4 i386 psABI, but it
> deliberately doesn't define the raw syscall interface, only the
> each-syscall-is-a-C-function one implemented by the C library,
> and that interface doesn't suffer from the current issue.
> 
> IOW, the kernel may not be at fault if user-space code invokes int
> $0x80 directly and then sees clobbered registers.

Ah, that, indeed is the issue.  As far as C is concerned,
the call is NOT a call, but a bit of asm.  If the asm is
correctly written the problem goes away, not because the
register is not modified, but because C is on notice that it
MIGHT be modified and thus not to count on it.

As a practical matter, ebx is used to pass arg1 to the
kernel so it must be changed by the asm code, the further
listing of it beyond the third ":" in the asm inline, will
cause the compiler to not rely on it being further
modified.  The same is true of all the registers used to
pass parameters.  (These are: arg1 ebx, arg2 ecx, arg3 edx,
arg4 esi, arg5 edi, and arg6 ebp.)

So, is there a problem?  Yes, neither the call stub macros
in asm/unistd.h nor those in glibc bother to list the used
registers beyond the third ":".  And, if I understand this
right, the glibc code to save ebx in another register
suffers from the false assumption that THAT register can be
clobbered, but this is only true if C sees the code as a
function, not an inline asm, but most system calls in glibc
are coded as inline asm, not separate functions (not to be
confused with the C inline, which is a separate function).

At least that is how I see it.  Comments?

-g

> 
> /Mikael
> -
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-- 
George Anzinger   [e-mail]
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