release/19.x: [llvm-exegesis] Use correct rseq struct size (#100804)

[llvmbot]

Backport 1e8df9e

Requested by: @boomanaiden154

[llvmbot]

@mgorny What do you think about merging this PR to the release branch?

[llvmbot]

@llvm/pr-subscribers-tools-llvm-exegesis

Author: None (llvmbot)

Changes

Backport 1e8df9e

Requested by: @boomanaiden154

---

Full diff: https://github.com/llvm/llvm-project/pull/100896.diff

1 Files Affected:

• (modified) llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp (+12-1)

diff --git a/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp b/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
index ed53f8fabb175..adee869967d98 100644
--- a/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
+++ b/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp
@@ -466,9 +466,20 @@ class SubProcessFunctionExecutorImpl
 // segfaults in the program. Unregister the rseq region so that we can safely
 // unmap it later
 #ifdef GLIBC_INITS_RSEQ
+    unsigned int RseqStructSize = __rseq_size;
+
+    // Glibc v2.40 (the change is also expected to be backported to v2.35)
+    // changes the definition of __rseq_size to be the usable area of the struct
+    // rather than the actual size of the struct. v2.35 uses only 20 bytes of
+    // the 32 byte struct. For now, it should be safe to assume that if the
+    // usable size is less than 32, the actual size of the struct will be 32
+    // bytes given alignment requirements.
+    if (__rseq_size < 32)
+      RseqStructSize = 32;
+
     long RseqDisableOutput =
         syscall(SYS_rseq, (intptr_t)__builtin_thread_pointer() + __rseq_offset,
-                __rseq_size, RSEQ_FLAG_UNREGISTER, RSEQ_SIG);
+                RseqStructSize, RSEQ_FLAG_UNREGISTER, RSEQ_SIG);
     if (RseqDisableOutput != 0)
       exit(ChildProcessExitCodeE::RSeqDisableFailed);
 #endif // GLIBC_INITS_RSEQ

[github-actions]

@boomanaiden154 (or anyone else). If you would like to add a note about this fix in the release notes (completely optional). Please reply to this comment with a one or two sentence description of the fix. When you are done, please add the release:note label to this PR.