[llvm-exegesis] Use correct rseq struct size (#100804)

Glibc v2.40 changes the definition of __rseq_size to the usable area of
the struct rather than the actual size of the struct to accommodate
users trying to figure out what features can be used. This change breaks
llvm-exegesis trying to disable rseq as the size registered in the
kernel is no longer equal to __rseq_size. This patch adds a check to see
if __rseq_size is less than 32 bytes and uses 32 as a value if it is
given alignment requirements.

Fixes #100791.

Author: boomanaiden154

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