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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 - Incorrect DWARF expressions produced with SafeStack"
href="https://bugs.llvm.org/show_bug.cgi?id=44585">44585</a>
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<th>Summary</th>
<td>Incorrect DWARF expressions produced with SafeStack
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</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>
<tr>
<th>Component</th>
<td>DebugInfo
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>leonardchan@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>jdevlieghere@apple.com, keith.walker@arm.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
</td>
</tr></table>
<p>
<div>
<pre>Given the following snippet:
```
#include <stdio.h>
#include <string>
int main(int argc, char* argv[]) {
std::string value = "asdf";
printf("%s\n", value.c_str());
return 0;
}
```
And compiling with (tip of tree) clang: `clang++ -g -fsanitize=safe-stack
/tmp/test.cpp`, we are unable to identify symbols:
```
$ gdb a.out
(gdb) start
5 int main(int argc, char* argv[]) {
(gdb) n
6 std::string value = "asdf";
(gdb)
7 printf("%s\n", value.c_str());
(gdb) p value
Cannot access memory at address 0x1
```
We can assert `p value` prints "asdf" when compiling without SafeStack. A
dwarfdump shows that "value" looks like:
```
0x00001ab0: DW_TAG_variable
DW_AT_location (0x00000000:
[0x0000000000000013, 0x000000000000003a): DW_OP_breg2
RCX-32
[0x000000000000003a, 0x0000000000000128): DW_OP_breg6
RBP-40, DW_OP_constu 0x20, DW_OP_minus, DW_OP_deref)
DW_AT_name ("value")
DW_AT_decl_file ("/tmp/test.cpp")
DW_AT_decl_line (5)
DW_AT_type (0x00000741 "string")
```
The `DW_OP_breg2 RCX-32` seems correct, but the `DW_OP_breg6 RBP-40,
DW_OP_constu 0x20, DW_OP_minus, DW_OP_deref` should be something like
`DW_OP_breg6 RBP-40, DW_OP_deref, DW_OP_constu 0x20, DW_OP_minus` to reflect:
```
movq -40(%rbp), %rax
addq $-40, %rax
```
A bisect was able to determine that <a href="https://reviews.llvm.org/D68945">https://reviews.llvm.org/D68945</a> introduced
this bug. The issue was initially addressed in <a href="https://reviews.llvm.org/D65172">https://reviews.llvm.org/D65172</a>
but was re-introduced.</pre>
</div>
</p>
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