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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 - [CFI] Codegen performs argument expansion after CFI check - invalidating CFI."
href="https://bugs.llvm.org/show_bug.cgi?id=35353">35353</a>
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<th>Summary</th>
<td>[CFI] Codegen performs argument expansion after CFI check - invalidating CFI.
</td>
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<th>Product</th>
<td>clang
</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>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>mitchphillips@outlook.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Clang's codegen is generating code in the following order:
1. CFI check for vcall.
2. Evaluation of arguments.
3. Execution of vcall.
This severely undermines the effectiveness of CFI, as non-sequential control
flow instructions should not be present between the CFI check and the execution
of the protected indirect call/jump.
The most common instance of this issue is when we have a vcall which has at
least one argument provided by a function-returned temporary. Even if the
function which is providing the argument is a direct call, we cannot guarantee
that the register(s) used to make the protected indirect call/jump are not
clobbered through the call. Often, the compiler will save the register(s) used
by the protected call/jump to a scratch register.
This issue affects approximately 6,932 instructions in the Chrome browser
binary, representing 41.8% of all "unexpected unprotected" indirect CF
instructions.
This issue is revealed by llvm-cfi-verify. Please see below for an example:
$ clang++ -flto -fsanitize=cfi -fvisibility=hidden -g a3.cc
$ llvm-cfi-verify a.out
----------------- Begin Instruction -----------------
FAIL_KNOWN_ISSUE 0x40067c: callq *%r14
0x40067c = /tmp/a3.cc:9:6 (main)
-----------------------------------------------------
Total Indirect CF Instructions: 1
Expected Protected: 0 (0.00%)
Unexpected Protected: 0 (0.00%)
Expected Unprotected: 0 (0.00%)
Unexpected Unprotected (BAD): 1 (100.00%)
$ cat a3.cc
struct A {
virtual void f(int) {}
};
int x() { return 0; }
int main() {
A* a = new A();
a->f(x()); // Should be CFI protected - x() is executed between CFI check
and execution.
}
$ objdump -d a.out
<..snip..>
40064f: 48 b9 60 07 40 00 00 movabs $0x400760,%rcx
400656: 00 00 00
400659: 48 83 c1 10 add $0x10,%rcx
40065d: 48 89 5d e8 mov %rbx,-0x18(%rbp)
400661: 48 8b 5d e8 mov -0x18(%rbp),%rbx
400665: 48 8b 03 mov (%rbx),%rax
400668: 48 39 c8 cmp %rcx,%rax
40066b: 74 02 je 40066f <main+0x4f> # cfi check
branch
40066d: 0f 0b ud2 #
cfi-failure
40066f: 4c 8b 30 mov (%rax),%r14
400672: e8 99 ff ff ff callq 400610 <_Z1xv> # direct
call, can clobber %r14
400677: 48 89 df mov %rbx,%rdi
40067a: 31 f6 xor %esi,%esi
40067c: 41 ff d6 callq *%r14 #
instruction that should be protected
<..snip..></pre>
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