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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 handling of relocations to unresolved weak references with -pie not consistent"
href="https://bugs.llvm.org/show_bug.cgi?id=50759">50759</a>
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<th>Summary</th>
<td>LLD handling of relocations to unresolved weak references with -pie not consistent
</td>
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<th>Product</th>
<td>lld
</td>
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<th>Version</th>
<td>unspecified
</td>
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<tr>
<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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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>smithp352@googlemail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, smithp352@googlemail.com
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<p>
<div>
<pre>When we do a -pie link with no shared libraries present LLD can be inconsistent
with its resolution of unresolved weak-references.
Consider the example below (I'm using aarch64 but it should reproduce on other
targets):
// weak.c
extern __attribute__((weak)) int weak_reference;
__attribute__((visibility("hidden"))) int* address_of_weak_reference =
&weak_reference;
void _start() {
if (&weak_reference)
weak_reference = 1;
if (address_of_weak_reference)
*address_of_weak_reference = 1;
}
clang -c weak.c -fpie --target=aarch64-linux-gnu -ffreestanding -nostdlib
ld.lld -pie weak.o -o weak-dyn
readelf --relocs weak-dyn
Relocation section '.rela.dyn' at offset 0x278 contains 1 entries:
Offset Info Type Symbol's Value
Symbol's Name + Addend
0000000000020390 0000000100000401 R_AARCH64_GLOB_DAT 0000000000000000
weak_reference + 0
In the object file there are GOT generating relocations for the references to
weak_reference and a R_AARCH64_ABS64 relocation from the .data section. LLD is
relocating the GOT reference to weak_reference but is statically resolving the
R_AARCH64_ABS64 to the undefined weak_reference to 0
ld.bfd does what I'd expect for this case as -pie is expected to be used with a
dynamic linker:
Relocation section '.rela.dyn' at offset 0x278 contains 2 entries:
Offset Info Type Symbol's Value
Symbol's Name + Addend
0000000000010fe0 0000000300000401 R_AARCH64_GLOB_DAT 0000000000000000
weak_reference + 0
0000000000011000 0000000300000101 R_AARCH64_ABS64 0000000000000000
weak_reference + 0
A second part to this is motivated by the linux kernel KASLR configuration that
uses a combination of -fpie and linking -pie with only static libraries and
with a linker script including something like:
# weak.lds
/DISCARD/ : {
*(.interp .dynamic)
*(.dynsym .dynstr .hash .gnu.hash)
}
If we use LLD with this linker script extract the dynamic symbol table will be
removed, leaving the R_AARCH64_GLOB_DAT with a symbol index of 0. This used to
give a warning, but no longer does so.
ld.lld -pie weak.o -o weak --script=weak.lds
Relocation section '.rela.dyn' at offset 0x10000 contains 1 entries:
Offset Info Type Symbol's Value
Symbol's Name + Addend
0000000000000058 0000000000000401 R_AARCH64_GLOB_DAT 0
In this case I think there is a strong argument to statically resolve all
dynamic relocations to unresolved weak references to 0 as a R_AARCH64_GLOB_DAT
to symbol idx 0 is arguably ill-formed.
Perhaps we can even error if there are any dynamic relocations expecting a
symbol when the .dynsym is discarded.</pre>
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