Re: STT_FUNC for assembler checksum and semaphore ops" in git-x86

From: John Reiser
Date: Thu Jan 10 2008 - 23:18:30 EST


Andi Kleen wrote:
> On Thu, Jan 10, 2008 at 04:59:52PM -0800, John Reiser wrote:
>
>>Andi Kleen wrote:
>>
>>>But actually checking the default implementation in linkage.h already
>>>implements size: [snip]
>>
>>>Are you sure it doesn't work? Your patch should be not needed. If it's
>>>still wrong then just ENDPROCs() need to be added.
>>
>>The ENDPROCs() were not used everywhere. Some code used just END() instead,
>>while other code used nothing. um/sys-i386/checksum.S didn't #include
>
>
> END() is fine too since it contains .size too:
>
> #ifndef END
> #define END(name) \
> .size name, .-name
> #endif
>
>
>>diff --git a/arch/x86/lib/semaphore_32.S b/arch/x86/lib/semaphore_32.S
>>index 444fba4..e2c6e0d 100644
>>--- a/arch/x86/lib/semaphore_32.S
>>+++ b/arch/x86/lib/semaphore_32.S
>>@@ -49,7 +49,7 @@ ENTRY(__down_failed)
>> ENDFRAME
>> ret
>> CFI_ENDPROC
>>- END(__down_failed)
>>+ ENDPROC(__down_failed)
>
>
> I don't think these change makes sense given the definition of END()
> shown above.
>
> The only change that would make sense is adding END() (or ENDPROC())
> to a function that doesn't have either of them yet.

No. The pseudo op ".type name, @function" appears only in ENDPROC;
it does not appear in END. So changing END to ENDPROC *does* alter
the Elf32_Sym for 'name'. Just END produces STT_NOTYPE; ENDPROC
produces STT_FUNC. A static analysis tool can get the info it wants
much more easily if all subroutines are marked as STT_FUNC.
In theory the tool could sort the symbols, notice the disjoint
coverage of the address space by the .size intervals of consecutive
symbols that are the targets of a CALL instruction, and thus deduce
that ".type foo, @function" *should* have been specified. But this
is a heuristic, and it fails on boundaries where assembly code is
invoked via trap, interrupt, or exception (anything other than CALL).
Instead, specify STT_FUNC for each subroutine in the first place.
That requires .type, which means ENDPROC (not END) from linux/linkage.h.

--
John Reiser, jreiser@xxxxxxxxxxxx
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