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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - MemorySanitizer va arg helpers are broken if function has many parameters before vararg part."
href="https://llvm.org/bugs/show_bug.cgi?id=27646">27646</a>
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<th>Summary</th>
<td>MemorySanitizer va arg helpers are broken if function has many parameters before vararg part.
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Miscellaneous Instrumentation passes
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>koriakin@0x04.net
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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<p>
<div>
<pre>The x86_64 and aarch64 vararg helpers in memorysanitizer pass currently assume
no non-vararg argument ends up in the overflow area - if that's not true, the
data stored to __msan_va_arg_tls will be misaligned with the overflow pointer
computed by va_start. Here's a testcase for x86_64:
#include <stdio.h>
#include <stdarg.h>
int passarg(int a, int b, int c, int d, int e, int f, int g, ...) {
va_list v;
va_start(v, g);
int res = va_arg(v, int);
va_end(v);
return res;
}
int main() {
int undef;
int res = passarg(undef, undef, undef, undef, undef, undef, undef, 2);
if (res)
printf("%d\n", res);
return 0;
}
Compiling with -fsanitize=memory and running results in:
==22438==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x4889ef in main (/home/mwk/llvm/llvm/a.out+0x4889ef)
#1 0x7f78129fe70f in __libc_start_main (/usr/lib/libc.so.6+0x2070f)
#2 0x418e38 in _start (/home/mwk/llvm/llvm/a.out+0x418e38)
This is because va_arg effectively reads the shadow belonging to argument g.
aarch64 should be suspectible to the same issue, but I haven't checked it.
mips64 is even worse, since it assumes there's exactly one non-vararg argument.</pre>
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