Bug 26358 - --wrap: the special rule for "undefined symbol" is incompatible with -flto or relocatable links
Summary: --wrap: the special rule for "undefined symbol" is incompatible with -flto or...
Status: UNCONFIRMED
Alias: None
Product: binutils
Classification: Unclassified
Component: ld (show other bugs)
Version: unspecified
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
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Reported: 2020-08-08 23:49 UTC by Fangrui Song
Modified: 2020-08-08 23:57 UTC (History)
1 user (show)

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Description Fangrui Song 2020-08-08 23:49:02 UTC
The manpage says

> Only undefined references are replaced by the linker.  So, translation unit internal references to symbol are not resolved to "__wrap_symbol". In the next example, the call to "f" in "g" is not resolved to "__wrap_f".

The "undefined reference" rule can cause different views in non-LTO and LTO (or -r).

>a.c<<e cat
  int foo();
  int main() { return foo(); }
e
>b.c<<e cat
  int foo() { return 1; }
e
>c.c<<e cat
  int __wrap_foo() { return 2; }
e

gcc -fuse-ld=bfd a.c b.c c.c -Wl,--wrap=foo -o a
gcc -fuse-ld=bfd -flto a.c b.c c.c -Wl,--wrap=foo -o a.lto
gcc -fuse-ld=bfd a.c b.c c.c -r -o a.ro && gcc -fuse-ld=bfd a.ro -o rel

./a returns 2 while both a.lto and rel return 1.
# -fuse-ld=gold has the same issue.

In the non-LTO case, the symbol in a.o is essentially changed to __wrap_foo and then bound to the definition in c.o
In the relocatable case and the LTO case, foo is defined, so it is not renamed to __wrap_foo. The definition in b.o is picked.

I think the majority of use cases use --wrap for an external definition, so this strange behavior doesn't really matter. LLD's --wrap is implemented in such a way that LTO, non-LTO and relocatable links behave the same:

--wrap is handled after all other symbol resolution steps. Symbol definitions are not affected (i.e. the section order is unchanged: main,foo,__wrap_foo). The symbol table of each object file is mangled (foo -> __wrap_foo; __real_foo -> foo) so that all relocations to foo or __real_foo will be redirected.