[llvm-bugs] [Bug 44585] New: Incorrect DWARF expressions produced with SafeStack
via llvm-bugs
llvm-bugs at lists.llvm.org
Fri Jan 17 09:56:56 PST 2020
https://bugs.llvm.org/show_bug.cgi?id=44585
Bug ID: 44585
Summary: Incorrect DWARF expressions produced with SafeStack
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: DebugInfo
Assignee: unassignedbugs at nondot.org
Reporter: leonardchan at google.com
CC: jdevlieghere at apple.com, keith.walker at arm.com,
llvm-bugs at lists.llvm.org,
paul_robinson at playstation.sony.com
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 https://reviews.llvm.org/D68945 introduced
this bug. The issue was initially addressed in https://reviews.llvm.org/D65172
but was re-introduced.
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