3 files changed, 47 insertions, 16 deletions

diff --git a/libc/bionic/libc_logging.cpp b/libc/bionic/libc_logging.cpp
index b57e24297..d0172edaa 100644
--- a/libc/bionic/libc_logging.cpp
+++ b/libc/bionic/libc_logging.cpp
@@ -230,7 +230,6 @@ static void SendRepeat(Out& o, char ch, int count) {
 /* Perform formatted output to an output target 'o' */
 template <typename Out>
 static void out_vformat(Out& o, const char* format, va_list args) {
-    int caller_errno = errno;
     int nn = 0;
 
     for (;;) {
@@ -379,9 +378,6 @@ static void out_vformat(Out& o, const char* format, va_list args) {
         } else if (c == '%') {
             buffer[0] = '%';
             buffer[1] = '\0';
-        } else if (c == 'm') {
-            // syslog-like %m for strerror(errno).
-            str = strerror(caller_errno);
         } else {
             __assert(__FILE__, __LINE__, "conversion specifier unsupported");
         }
diff --git a/libc/bionic/syslog.cpp b/libc/bionic/syslog.cpp
index 29f892a20..d8b8b190d 100644
--- a/libc/bionic/syslog.cpp
+++ b/libc/bionic/syslog.cpp
@@ -14,6 +14,7 @@
  * limitations under the License.
  */
 
+#include <errno.h>
 #include <stdlib.h>
 #include <syslog.h>
 
@@ -47,6 +48,8 @@ void syslog(int priority, const char* fmt, ...) {
 }
 
 void vsyslog(int priority, const char* fmt, va_list args) {
+  int caller_errno = errno;
+
   // Check whether we're supposed to be logging messages of this priority.
   if ((syslog_priority_mask & LOG_MASK(LOG_PRI(priority))) == 0) {
     return;
@@ -71,5 +74,48 @@ void vsyslog(int priority, const char* fmt, va_list args) {
     android_log_priority = ANDROID_LOG_DEBUG;
   }
 
-  __libc_format_log_va_list(android_log_priority, log_tag, fmt, args);
+  // glibc's printf family support %m directly, but our BSD-based one doesn't.
+  // If the format string seems to contain "%m", rewrite it.
+  const char* log_fmt = fmt;
+  if (strstr(fmt, "%m") != NULL) {
+    size_t dst_len = 1024;
+    char* dst = reinterpret_cast<char*>(malloc(dst_len));
+    log_fmt = dst;
+
+    const char* src = fmt;
+    for (; dst_len > 0 && *src != '\0'; ++src) {
+      if (*src == '%' && *(src + 1) == 'm') {
+        // Expand %m.
+        size_t n = strlcpy(dst, strerror(caller_errno), dst_len);
+        if (n >= dst_len) {
+          n = dst_len;
+        }
+        dst += n;
+        dst_len -= n;
+        ++src;
+      } else if (*src == '%' && *(src + 1) == '%') {
+        // We need to copy pairs of '%'s so the %m test works.
+        if (dst_len <= 2) {
+          break;
+        }
+        *dst++ = '%'; --dst_len;
+        *dst++ = '%'; --dst_len;
+        ++src;
+      } else {
+        *dst++ = *src; --dst_len;
+      }
+    }
+    *dst = '\0';
+  }
+
+  // We can't let __libc_format_log do the formatting because it doesn't support
+  // all the printf functionality.
+  char log_line[1024];
+  vsnprintf(log_line, sizeof(log_line), log_fmt, args);
+
+  if (log_fmt != fmt) {
+    free(const_cast<char*>(log_fmt));
+  }
+
+  __libc_format_log(android_log_priority, log_tag, "%s", log_line);
 }
diff --git a/tests/libc_logging_test.cpp b/tests/libc_logging_test.cpp
index ef39d1c03..950161e78 100644
--- a/tests/libc_logging_test.cpp
+++ b/tests/libc_logging_test.cpp
@@ -176,14 +176,3 @@ TEST(libc_logging, lld_LLONG_MIN) {
   GTEST_LOG_(INFO) << "This test does nothing.\n";
 #endif // __BIONIC__
 }
-
-TEST(libc_logging, m) {
-#if defined(__BIONIC__)
-  char buf[BUFSIZ];
-  errno = EBADF;
-  __libc_format_buffer(buf, sizeof(buf), "<%m>");
-  EXPECT_STREQ("<Bad file number>", buf);
-#else // __BIONIC__
-  GTEST_LOG_(INFO) << "This test does nothing.\n";
-#endif // __BIONIC__
-}