[Bug ld/16715] -Bsymbolic breaks PMF comparison under ARM

[thomas at kdab dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=16715

--- Comment #2 from Thomas McGuire <thomas at kdab dot com> ---
Attached a C-only testcase (forgot to rename the files from .cpp to .c, but I
don't think that matters). The testcase simply outputs the function pointer of
a function in a shared library, and one can see that the address is not the
same.

Compile with:
gcc -fPIC -shared -Wall -o libshared.so -Wl,-Bsymbolic shared.cpp
gcc -fPIE -Wall -o main main.cpp -L. -lshared

The output on my machine is:
# ./main 
0x8518
0x2ac595cc

The root of the problem seems to be that main gets the following undefined
symbol:
   Num:    Value  Size Type    Bind   Vis      Ndx Name           
    14: 0000853c     0 FUNC    GLOBAL DEFAULT  UND testFunction()

This is a special hack - an undefined symbol that nevertheless has a value! See
http://www.airs.com/blog/archives/42 for details on how that hack is supposed
to work.
The idea is to use a different symbol value depending on the relocation type -
a relocation for R_ARM_JUMP_SLOT should always resolve to the local PLT, but a
relocation of type R_ARM_GLOB_DAT should use the symbol value defined in main.
The problem is that libshared.so doesn't contain a R_ARM_GLOB_DAT relocation
when taking the address of the function, but a R_ARM_RELATIVE relocation, and
will therefore never resolve the function address to the value in main.
I guess this is a consequence of using -Bsymbolic - when taking the address of
a function, it should still go through the GOT, with a R_ARM_GLOB_DAT
relocation, despite -Bsymbolic being set.

Or at least that is what I think is the cause of the problem, I am by no means
a Linker export, I only recently started being interested in this topic.

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