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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - LLD does not maintain address equivalence with DSO for ifunc defined in PIE executable."
href="https://bugs.llvm.org/show_bug.cgi?id=40501">40501</a>
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<th>Summary</th>
<td>LLD does not maintain address equivalence with DSO for ifunc defined in PIE executable.
</td>
</tr>
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<th>Product</th>
<td>lld
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>ELF
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>peter.smith@linaro.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, peter.smith@linaro.org
</td>
</tr></table>
<p>
<div>
<pre>When -fpie is used, the address of an ifunc defined in an executable should be
the same when referenced by a shared library. This can be shown to be the case
for ld.gold and ld.bfd but not ld.lld. I've tried this on x86_64 and AArch64.
Consider the example:
// FILE dso.c
typedef void fptr(void);
extern void fff(void);
fptr *global_fptr = &fff;
// FILE main.c
#include <stdio.h>
static void fff_impl() {
printf("fff_impl()\n");
}
void *fff_resolver() {
return (void *)&fff_impl;
}
__attribute__((ifunc("fff_resolver"))) void fff();
typedef void fptr(void);
fptr *local_fptr = fff;
extern fptr *global_fptr;
int main()
{
printf("local %p global %p\n", local_fptr, global_fptr);
return 0;
}
With:
clang -fpic dso.c -o dso.so --shared
bin/clang -fpie -pie main.c dso.so -o main.exe
LD_LIBRARY_PATH=. ./main.exe
With -fuse-ld=gold:
local 0x559bbd215810 global 0x559bbd215810
With -fuse-ld=bfd:
local 0x562c653647f0 global 0x562c653647f0
With -fuse-ld=lld
local 0x55b9fda0e0f0 global 0x55b9fda0e140
On AArch64 the problem is that LLD generates a GOT entry for fff (local_fptr)
that points to the PLT entry for fff. If we call local_fptr() we get the
correct result after going through the PLT, but global_fptr gets the address of
fff_impl from the dynamic loader (fff_impl is the result of the
ifunc_resolver).
PR40474 has some more information about what ld.bfd and ld.gold do for AArch64
in this case.</pre>
</div>
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