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Re: [PATCH][RFC] Explicitly set ELF symbol size to zero for undefined symbols -- libbfd, gold


On Mon, Jul 14, 2008 at 11:48:25AM +0100, Simon Baldwin wrote:
> This patch explicitly sets the size of undefined ELF symbols to zero for
> linker output.
> 
> Currently, the size of an undefined ELF symbol is copied out of the object
> file or DSO that supplies the symbol, on linking.  This size is unreliable,
> for example in the case of two DSOs, one linking to the other.  The lower-
> level DSO could make an ABI-preserving change that alters the symbol size,
> with no hard requirement to rebuild the higher-level DSO.  And if the higher-
> level DSO is rebuilt, tools that monitor file checksums will register a
> change due to the altered size of the undefined symbol, even though nothing
> else about the higher-level DSO has altered.  This can lead to unnecessary
> and undesirable rebuild and change cascades in checksum-based systems.
> 
> Confirmed test parity between pre- and post-patch source trees.
> 
> Any objections?  Thanks.

I feel uncomfortable about this patch, simply because it loses
information.  Not that I can think of a solid reason why this would
matter.  SHN_COMMON symbols need both size and value, and SHN_UNDEF
can't lose a non-zero value (plt symbol hint to ld.so), but SHN_UNDEF
size doesn't seem to be that useful.  However, people do weird things
with linkers..

I'd be much happier if you modified elf_link_add_object_symbols to
zero size on reading undefined symbols from dynamic objects.  In fact,
there seems to be code there already that would do this.  See
"/* Remember the symbol size if it isn't undefined.  */".  Why isn't
this code effective?

-- 
Alan Modra
Australia Development Lab, IBM


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