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[sanitizer] Don't TestPTrace() if SPARC; don't give up if internal_fork() fails (#152072)

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Fixes corner cases of https://github.com/llvm/llvm-project/pull/151406:
- Don't run TestPTrace() on SPARC, because internal_fork() on SPARC
actually calls __fork(). We can't safely __fork(), because it's possible
seccomp has been configured to disallow fork() but allow clone().
- if internal_fork() fails for whatever reason, we shouldn't give up. It
is strictly worse to give up early than to attempt StopTheWorld.

Also updates some comments/TODOs.
This commit is contained in:
Thurston Dang 2025-08-05 16:31:35 -07:00 committed by GitHub
parent b8eb61adc9
commit 435b8b51dc
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1 changed files with 29 additions and 7 deletions
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36
compiler-rt/lib/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp
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@ -405,10 +405,21 @@ struct ScopedSetTracerPID {
// This detects whether ptrace is blocked (e.g., by seccomp), by forking and
// then attempting ptrace.
// This separate check is necessary because StopTheWorld() creates a child
// process with a shared virtual address space and shared TLS, and therefore
// This separate check is necessary because StopTheWorld() creates a thread
// with a shared virtual address space and shared TLS, and therefore
// cannot use waitpid() due to the shared errno.
static void TestPTrace() {
# if SANITIZER_SPARC
// internal_fork() on SPARC actually calls __fork(). We can't safely fork,
// because it's possible seccomp has been configured to disallow fork() but
// allow clone().
Report("WARNING: skipping TestPTrace() because this is SPARC\n");
Report(
"If seccomp blocks ptrace, LeakSanitizer may hang without further "
"notice\n");
Report(
"If seccomp does not block ptrace, you can safely ignore this warning\n");
# else
// Heuristic: only check the first time this is called. This is not always
// correct (e.g., user manually triggers leak detection, then updates
// seccomp, then leak detection is triggered again).
@ -417,35 +428,46 @@ static void TestPTrace() {
return;
checked = true;
// We hope that fork() is not too expensive, because of copy-on-write.
// Hopefully internal_fork() is not too expensive, thanks to copy-on-write.
// Besides, this is only called the first time.
// Note that internal_fork() on non-SPARC Linux actually calls
// SYSCALL(clone); thus, it is reasonable to use it because if seccomp kills
// TestPTrace(), it would have killed StopTheWorld() anyway.
int pid = internal_fork();
if (pid < 0) {
int rverrno;
if (internal_iserror(pid, &rverrno)) {
if (internal_iserror(pid, &rverrno))
Report("WARNING: TestPTrace() failed to fork (errno %d)\n", rverrno);
}
internal__exit(-1);
// We don't abort the sanitizer - it's still worth letting the sanitizer
// try.
return;
}
if (pid == 0) {
// Child subprocess
// TODO: consider checking return value of internal_ptrace, to handle
// SCMP_ACT_ERRNO. However, be careful not to consume too many
// resources performing a proper ptrace.
internal_ptrace(PTRACE_ATTACH, 0, nullptr, nullptr);
internal__exit(0);
} else {
int wstatus;
internal_waitpid(pid, &wstatus, 0);
// Handle SCMP_ACT_KILL
if (WIFSIGNALED(wstatus)) {
VReport(0,
"Warning: ptrace appears to be blocked (is seccomp enabled?). "
"WARNING: ptrace appears to be blocked (is seccomp enabled?). "
"LeakSanitizer may hang.\n");
VReport(0, "Child exited with signal %d.\n", WTERMSIG(wstatus));
// We don't abort the sanitizer - it's still worth letting the sanitizer
// try.
}
}
# endif
}
void StopTheWorld(StopTheWorldCallback callback, void *argument) {