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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 can create GNU hash section with 0 buckets (incompatible with Android)"
href="https://bugs.llvm.org/show_bug.cgi?id=36537">36537</a>
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<th>Summary</th>
<td>LLD can create GNU hash section with 0 buckets (incompatible with Android)
</td>
</tr>
<tr>
<th>Product</th>
<td>lld
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>ELF
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>peter.smith@linaro.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>I've found that LLD cannot link various Android executables due to the dynamic
linker not liking GNU Hash Sections with nbuckets == 0. It gives the error
message: empty/missing DT_HASH/DT_GNU_HASH in \"%s\" "
"(new hash type from the future?)"
<a href="https://android.googlesource.com/platform/bionic/+/master/linker/linker.cpp#3469">https://android.googlesource.com/platform/bionic/+/master/linker/linker.cpp#3469</a>
Tracing this back through the code, this can occur when all the symbols are in
the lower stable partition in GnuHashTableSection::addSymbols() as this will
bail out before NBuckets is set.
When all the symbols in an executable are in the lower partition in:
std::vector<SymbolTableEntry>::iterator Mid =
std::stable_partition(V.begin(), V.end(), [](const SymbolTableEntry &S) {
// Shared symbols that this executable preempts are special. The
dynamic
// linker has to look them up, so they have to be in the hash table.
if (auto *SS = dyn_cast<SharedSymbol>(S.Sym))
return SS->CopyRelSec == nullptr && !SS->NeedsPltAddr;
return !S.Sym->isDefined();
});
if (Mid == V.end())
return;
// We chose load factor 4 for the on-disk hash table. For each hash
// collision, the dynamic linker will compare a uint32_t hash value.
// Since the integer comparison is quite fast, we believe we can make
// the load factor even larger. 4 is just a conservative choice.
NBuckets = std::max<size_t>((V.end() - Mid) / 4, 1);
To reproduce on Linux I've found that I need to compile and link with -fpie
-pie.
For example:
#include <stdio.h>
int main(void) {
printf("Hello World\n");
return 0;
}
clang hashtest.c -fpie -Wl,--hash-style=gnu -o hashtest.exe
-fuse-ld=/linaro/upstream/buildclang/bin/ld.lld -pie
llvm-readobj -gnu-hash-table hashtest.exe
File: hashtest.exe
Format: ELF64-x86-64
Arch: x86_64
AddressSize: 64bit
LoadName:
GnuHashTable {
Num Buckets: 0
First Hashed Symbol Index: 8
Num Mask Words: 1
Shift Count: 6
Bloom Filter: [0x0]
Buckets: []
Values: []
}
I'm not convinced that the bionic loader is correct to reject the executable
becasue nbuckets is 0. It could treat that as an empty table and just skip it.
However I think that LLD should probably remove the Hash Table section
completely if it is empty.</pre>
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