[Bug target/19520] protected function pointer doesn't work right

[thiago at kde dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19520

--- Comment #26 from Thiago Macieira <thiago at kde dot org> 2012-01-18 13:28:05 UTC ---
ld *can* link, it just chooses not to.

$ cat > foo.c
__attribute__((visibility("protected")))
void * foo (void) { return (void *)foo; }

$ gcc -fPIC -shared foo.c           
/usr/bin/ld: /tmp/cclrufLV.o: relocation R_X86_64_PC32 against protected symbol
`foo' can not be used when making a shared object
/usr/bin/ld: final link failed: Bad value
collect2: ld returned 1 exit status

$ gcc -Wl,-Bsymbolic-functions -fPIC -shared foo.c && echo success
success
$ cat > empty.dynlist                                                     
{ "__this_symbol_isnt_present__"; };
$ gcc -Wl,--dynamic-list,empty.dynlist -fPIC -shared foo.c && echo success
success

I also cannot confirm that icc does anything different:
$ icc -fPIC -shared foo.c
ld: /tmp/iccf15gTK.o: relocation R_X86_64_PC32 against protected symbol `foo'
can not be used when making a shared object
ld: final link failed: Bad value
$ icc -O3 -S -o /dev/stdout -fPIC -shared foo.c | grep -A4 foo:
foo:
..B1.1:                         # Preds ..B1.0
..___tag_value_foo.1:                                           #2.19
        lea       foo(%rip), %rax                               #2.36
        ret                                                     #2.36

What's more, if you actually do compile the following program into a shared
library, it succeeds:
$ cat > foo.S
        .text
        .globl  foo
        .protected      foo
        .type   foo, @function
foo:
        movq      foo@GOTPCREL(%rip), %rax
        ret
$ gcc -shared foo.S && echo success
success

But the resulting shared object has the following (extracted from eu-readelf):
Relocation section [ 5] '.rela.dyn' for section [ 0] '' at offset 0x230
contains 1 entry:
  Offset              Type            Value               Addend Name
  0x0000000000200330  X86_64_GLOB_DAT 0x0000000000000248      +0 foo

    2: 0000000000000248      0 FUNC    GLOBAL PROTECTED      6 foo

Now we introduce a third component to this discussion: the dynamic linker. What
will it do?

This has become a decision, not a bug: what should the compiler do when taking
the address of a function when said function is under protected visibility.
Both solutions are technically correct and would load the same function address
under the correct circumstances. 

The compiler is also taking on the "protected" visibility to the letter (at
least, according to its own definition of so):

    "protected"
          Protected visibility is like default visibility except that it
          indicates that references within the defining module will
          bind to the definition in that module.  That is, the declared
          entity cannot be overridden by another module.

Since the symbol was marked as "protected" in the symbol table, it's expected
that the linker and dynamic linker will bind it locally. That being the case,
the compiler can optimise for that fact. It can calculate what value would be
placed in the GOT entry and load that instead. That's the LEA instruction.

The linker, however, mandates that the address to symbol should not be loaded
directly, but only through the GOT. This is necessary because the psABI
requires that the function address resolve to the PLT entry found in the
position-dependent executable. If the executable takes the address of this
global (but protected) symbol, it will hardcode the address to its own address
space, forcing other ELF modules to follow suit.

Finally, what does the dynamic linker do when an "entity (that) cannot be
overridden by another module" is overridden by another module? The glibc 2.14
loader will resolve the GOT entry's relocation to the executable's PLT stub,
even if the symbol in question has protected visibility. Other loaders might
work differently.

As it stands, the psABI requires that the address to a protected function be
loaded through the GOT, even though the compiler thinks it knows what the
address will be.

However, I really wish the compiler *not* to change its behaviour for PIC code,
but instead change its behaviour for ELF position-dependent executables. I am
asking for a change in the psABI and requesting that the loading of function
addresses for "default" visibility symbols (not protected!) should be done via
the GOT. In other words, I'm asking that we optimise for shared libraries, not
for executables.

Versions:
GCC: 4.6.0
ld: 2.21.51.0.6-6.fc15 20110118
ICC: 12.1.0 20111011