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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [AArch64] Cross-DSO CFI doesn't work with ShadowCallStack and mcount instrumentation"
href="https://bugs.llvm.org/show_bug.cgi?id=44094">44094</a>
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<th>Summary</th>
<td>[AArch64] Cross-DSO CFI doesn't work with ShadowCallStack and mcount instrumentation
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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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
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>peter@pcc.me.uk
</td>
</tr>
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<th>Reporter</th>
<td>samitolvanen@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, ndesaulniers@google.com
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<p>
<div>
<pre>Compiling any C file with cross-DSO CFI, ShadowCallStack, and mcount
instrumentation all enabled, and linking it into native code with LLD results
in "LLVM ERROR: Must reserve x18 to use shadow call stack", even though the
code was compiled with -ffixed-x18.
$ clang --version | grep version
clang version 10.0.0 (<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
73429126c91c2065c6f6ef29b3eec1b7798502bb)
$ ld.lld --version
LLD 10.0.0 (<a href="https://github.com/llvm/llvm-project.git">https://github.com/llvm/llvm-project.git</a>
73429126c91c2065c6f6ef29b3eec1b7798502bb) (compatible with GNU linkers)
$ cat build.sh
#!/bin/bash
cat /dev/null > main.c
clang \
-target aarch64-linux-gnu \
-flto=thin \
-fsanitize=cfi \
-fvisibility=default \
-fsanitize-cfi-cross-dso \
-fsanitize=shadow-call-stack \
-ffixed-x18 \
-pg \
-c main.c \
-o main.o
ld.lld -r -o all.o main.o
$ ./build.sh
LLVM ERROR: Must reserve x18 to use shadow call stack
Looking at the IR code in main.o, the interesting part is the __cfi_check_fail
function generated by the compiler:
define weak_odr hidden void @__cfi_check_fail(i8* %0, i8* %1) #0 {
%3 = alloca i8*, align 8
%4 = alloca i8*, align 8
store i8* %0, i8** %3, align 8, !nosanitize !7
store i8* %1, i8** %4, align 8, !nosanitize !7
%5 = load i8*, i8** %3, align 8, !nosanitize !7
%6 = load i8*, i8** %4, align 8, !nosanitize !7
%7 = icmp ne i8* %5, null, !nosanitize !7
br i1 %7, label %9, label %8, !nosanitize !7
...
attributes #0 = { shadowcallstack
"instrument-function-entry-inlined"="\01_mcount" "min-legal-vector-width"="0"
"no-jump-tables"="false" }
The function is generated with the shadowcallstack attribute, but it doesn't
have a target-features attribute with +reserve-x18.
The test case links again if I remove any of -fsanitize-cfi-cross-dso, -pg, or
fsanitize=shadow-call-stack from the compiler flags.</pre>
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