[Sanitizer] Option to fallback to stderr if unable to open logfile #158687

ellishg · 2025-09-15T17:00:28Z

Add the santizier option log_fallback_to_stderr which will set the logpath to stderr if there is an error with the provided logpath. We've seen this happen when process A has write permission to the logpath, but process B does not. In this case, we'd like process B to fallback to writing to stderr, rather than being killed.

llvmbot · 2025-09-15T17:01:05Z

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Ellis Hoag (ellishg)

Changes

Add the santizier option log_fallback_to_stderr which will set the logpath to stderr if there is an error with the provided logpath. We've seen this happen when process A has write permission to the logpath, but process B does not. In this case, we'd like process B to fallback to writing to stderr, rather than being killed.

Full diff: https://github.com/llvm/llvm-project/pull/158687.diff

4 Files Affected:

(modified) compiler-rt/lib/sanitizer_common/sanitizer_file.cpp (+29-8)
(modified) compiler-rt/lib/sanitizer_common/sanitizer_file.h (+3)
(modified) compiler-rt/lib/sanitizer_common/sanitizer_flags.inc (+2)
(modified) compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp (+3)

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
index 9236a458cdb0e..4e691d0768c52 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
@@ -36,9 +36,16 @@ void RawWrite(const char *buffer) {
 
 void ReportFile::ReopenIfNecessary() {
   mu->CheckLocked();
-  if (fd == kStdoutFd || fd == kStderrFd) return;
+  if (!lastOpenFailed)
+    if (fd == kStdoutFd || fd == kStderrFd)
+      return;
 
   uptr pid = internal_getpid();
+  if (lastOpenFailed && fd_pid != pid) {
+    fd = kInvalidFd;
+    lastOpenFailed = false;
+  }
+
   // If in tracer, use the parent's file.
   if (pid == stoptheworld_tracer_pid)
     pid = stoptheworld_tracer_ppid;
@@ -48,8 +55,7 @@ void ReportFile::ReopenIfNecessary() {
     // process, close it now.
     if (fd_pid == pid)
       return;
-    else
-      CloseFile(fd);
+    CloseFile(fd);
   }
 
   const char *exe_name = GetProcessName();
@@ -71,12 +77,17 @@ void ReportFile::ReopenIfNecessary() {
     char errmsg[100];
     internal_snprintf(errmsg, sizeof(errmsg), " (reason: %d)\n", err);
     WriteToFile(kStderrFd, errmsg, internal_strlen(errmsg));
-    Die();
+    if (!common_flags()->log_fallback_to_stderr)
+      Die();
+    const char *errmsg2 = "ERROR: Fallback to stderr\n";
+    WriteToFile(kStderrFd, errmsg2, internal_strlen(errmsg2));
+    lastOpenFailed = true;
+    fd = kStderrFd;
   }
   fd_pid = pid;
 }
 
-static void RecursiveCreateParentDirs(char *path) {
+static void RecursiveCreateParentDirs(char *path, fd_t &fd) {
   if (path[0] == '\0')
     return;
   for (int i = 1; path[i] != '\0'; ++i) {
@@ -90,7 +101,13 @@ static void RecursiveCreateParentDirs(char *path) {
       WriteToFile(kStderrFd, path, internal_strlen(path));
       const char *ErrorMsgSuffix = "\n";
       WriteToFile(kStderrFd, ErrorMsgSuffix, internal_strlen(ErrorMsgSuffix));
-      Die();
+      if (!common_flags()->log_fallback_to_stderr)
+        Die();
+      path[i] = save;
+      const char *ErrorMsgSuffix2 = "ERROR: Fallback to stderr\n";
+      WriteToFile(kStderrFd, ErrorMsgSuffix2, internal_strlen(ErrorMsgSuffix2));
+      fd = kStderrFd;
+      return;
     }
     path[i] = save;
   }
@@ -166,7 +183,11 @@ void ReportFile::SetReportPath(const char *path) {
       WriteToFile(kStderrFd, path, 8);
       message = "...\n";
       WriteToFile(kStderrFd, message, internal_strlen(message));
-      Die();
+      if (!common_flags()->log_fallback_to_stderr)
+        Die();
+      const char *message2 = "ERROR: Fallback to stderr\n";
+      WriteToFile(kStderrFd, message2, internal_strlen(message2));
+      path = "stderr";
     }
   }
 
@@ -180,7 +201,7 @@ void ReportFile::SetReportPath(const char *path) {
     fd = kStdoutFd;
   } else {
     ParseAndSetPath(path, path_prefix, kMaxPathLength);
-    RecursiveCreateParentDirs(path_prefix);
+    RecursiveCreateParentDirs(path_prefix, fd);
   }
 }
 
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_file.h b/compiler-rt/lib/sanitizer_common/sanitizer_file.h
index bef2c842d9f24..f1a791e2aa3b1 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_file.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_file.h
@@ -43,6 +43,9 @@ struct ReportFile {
   // PID of the process that opened fd. If a fork() occurs,
   // the PID of child will be different from fd_pid.
   uptr fd_pid;
+  // Set to true if the last attempt to open the logfile failed, perhaps due to
+  // permission errors
+  bool lastOpenFailed = false;
 
  private:
   void ReopenIfNecessary();
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_flags.inc b/compiler-rt/lib/sanitizer_common/sanitizer_flags.inc
index c1e3530618c20..650a4580bbcf0 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_flags.inc
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_flags.inc
@@ -65,6 +65,8 @@ COMMON_FLAG(
     bool, log_to_syslog, (bool)SANITIZER_ANDROID || (bool)SANITIZER_APPLE,
     "Write all sanitizer output to syslog in addition to other means of "
     "logging.")
+COMMON_FLAG(bool, log_fallback_to_stderr, false,
+            "When set, fallback to stderr if we are unable to open log path.")
 COMMON_FLAG(
     int, verbosity, 0,
     "Verbosity level (0 - silent, 1 - a bit of output, 2+ - more output).")
diff --git a/compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp b/compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp
index af5187a0d3265..24adaaa4046c0 100644
--- a/compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp
+++ b/compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp
@@ -2,9 +2,11 @@
 
 // Case 1: Try setting a path that is an invalid/inaccessible directory.
 // RUN: not %run %t 2>&1 | FileCheck %s --check-prefix=ERROR1
+// RUN: %env_tool_opts=log_fallback_to_stderr=1 %run %t 2>&1 | FileCheck %s --check-prefixes=ERROR1,FALLBACK
 
 // Case 2: Try setting a path that is too large.
 // RUN: not %run %t A 2>&1 | FileCheck %s --check-prefix=ERROR2
+// RUN: %env_tool_opts=log_fallback_to_stderr=1 %run %t A 2>&1 | FileCheck %s --check-prefixes=ERROR2,FALLBACK
 
 #include <sanitizer/common_interface_defs.h>
 #include <stdio.h>
@@ -22,3 +24,4 @@ int main(int argc, char **argv) {
   }
   __sanitizer_set_report_path(buff);
 }
+// FALLBACK: Fallback to stderr

github-actions · 2025-09-15T17:05:21Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff origin/main HEAD --extensions inc,cpp,h -- compiler-rt/lib/sanitizer_common/sanitizer_file.cpp compiler-rt/lib/sanitizer_common/sanitizer_file.h compiler-rt/lib/sanitizer_common/sanitizer_flags.inc compiler-rt/test/sanitizer_common/TestCases/Posix/sanitizer_set_report_path_fail.cpp

⚠️
The reproduction instructions above might return results for more than one PR
in a stack if you are using a stacked PR workflow. You can limit the results by
changing origin/main to the base branch/commit you want to compare against.
⚠️

View the diff from clang-format here.

diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp b/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
index e8f219b94..916e7e79d 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_file.cpp
@@ -73,7 +73,7 @@ void ReportFile::ReopenIfNecessary() {
   fd = OpenFile(full_path, WrOnly, &err);
   if (fd == kInvalidFd) {
     bool fallback = common_flags()->log_fallback_to_stderr;
-    const char *ErrorMsgPrefix =
+    const char* ErrorMsgPrefix =
         fallback ? "WARNING: Can't open file, falling back to stderr: "
                  : "ERROR: Can't open file: ";
     WriteToFile(kStderrFd, ErrorMsgPrefix, internal_strlen(ErrorMsgPrefix));
@@ -89,7 +89,7 @@ void ReportFile::ReopenIfNecessary() {
   fd_pid = pid;
 }
 
-static void RecursiveCreateParentDirs(char *path, fd_t &fd) {
+static void RecursiveCreateParentDirs(char* path, fd_t& fd) {
   if (path[0] == '\0')
     return;
   for (int i = 1; path[i] != '\0'; ++i) {
@@ -99,7 +99,7 @@ static void RecursiveCreateParentDirs(char *path, fd_t &fd) {
     path[i] = '\0';
     if (!DirExists(path) && !CreateDir(path)) {
       bool fallback = common_flags()->log_fallback_to_stderr;
-      const char *ErrorMsgPrefix =
+      const char* ErrorMsgPrefix =
           fallback ? "WARNING: Can't create directory, falling back to stderr: "
                    : "ERROR: Can't create directory: ";
       WriteToFile(kStderrFd, ErrorMsgPrefix, internal_strlen(ErrorMsgPrefix));
@@ -185,7 +185,7 @@ void ReportFile::SetReportPath(const char *path) {
     uptr len = internal_strlen(path);
     if (len > sizeof(path_prefix) - 100) {
       bool fallback = common_flags()->log_fallback_to_stderr;
-      const char *message =
+      const char* message =
           fallback ? "WARNING: Path is too long, falling back to stderr: "
                    : "ERROR: Path is too long: ";
       WriteToFile(kStderrFd, message, internal_strlen(message));

ellishg · 2025-09-15T17:22:57Z

It seems that I cannot make the formatter happy. Currently the style is consistent with the rest of the file.

SharonXSharon · 2025-09-16T20:49:50Z

compiler-rt/lib/sanitizer_common/sanitizer_file.cpp

+  if (lastOpenFailed && fd_pid != pid) {
+    fd = kInvalidFd;
+    lastOpenFailed = false;
+  }
+


Could you add a comment explaining what this scenario is?

kyulee-com · 2025-09-17T22:02:52Z

compiler-rt/lib/sanitizer_common/sanitizer_file.cpp

-    Die();
+    if (!common_flags()->log_fallback_to_stderr)
+      Die();
+    const char *errmsg2 = "ERROR: Fallback to stderr\n";


The error message seems a bit awkward, as the code continues handling the situation gracefully instead of terminating immediately.
Ideally, I want to combine the error message under this flag into something like:

ERROR: Can't open file: {blah}. Falling back to stderr.

Alternatively, we could comment with WARNING: instead of ERROR: in a separate message, but combining them into one line as above seems slightly better.

kyulee-com · 2025-09-17T22:07:06Z

compiler-rt/lib/sanitizer_common/sanitizer_file.cpp

+    // process, mark fd as invalid so we attempt to open again.
+    CHECK_EQ(fd, kStderrFd);
+    fd = kInvalidFd;
+    lastOpenFailed = false;


It looks like this flag is being cleared here. I think it would be clearer to use a more descriptive name, such as fallbackTostderrActive.

kyulee-com

LGTM

ellishg requested a review from teresajohnson September 15, 2025 17:00

llvmbot added compiler-rt compiler-rt:sanitizer labels Sep 15, 2025

ellishg force-pushed the memprof-logs branch from 919a955 to 4ea07ef Compare September 15, 2025 17:07

[Sanitizer] Option to fallback to stderr if unable to open logfile

a0fbdc0

ellishg force-pushed the memprof-logs branch from 4ea07ef to a0fbdc0 Compare September 15, 2025 17:19

use flag value true

d94674c

SharonXSharon reviewed Sep 16, 2025

View reviewed changes

Add comment and check lastOpenFailed first

0c3b977

ellishg requested a review from kyulee-com September 17, 2025 18:39

kyulee-com reviewed Sep 17, 2025

View reviewed changes

emit warning message

8a8735b

kyulee-com approved these changes Sep 18, 2025

View reviewed changes

ellishg merged commit 8843111 into llvm:main Sep 22, 2025
8 of 9 checks passed

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[Sanitizer] Option to fallback to stderr if unable to open logfile #158687

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