Fix mte build breakage.

The strerror_buf is way too large, so instead of using a separate
buffer for just this string, reuse the already existing buffer.
Increase the buffer size to cover the maximum errno string.

Add a unit test to verify that none of the errno values are cut off
in the async_safe_format_buffer function when passing %m.

Bug: 274474681

Test: New unit test passes.
Test: Changing the buffer to a small value and verify that the test fails.
Change-Id: I4cb4652709582a8a6b958e12de5d923ec950e6b6

libc/async_safe/async_safe_log.cpp

@@ -254,7 +254,7 @@ static void out_vformat(Out& o, const char* format, va_list args) {
     bool alternate = false;
     size_t bytelen = sizeof(int);
     int slen;
-    char buffer[32]; /* temporary buffer used to format numbers */
+    char buffer[64];  // temporary buffer used to format numbers/format errno string
 
     char c;
 
@@ -345,7 +345,6 @@ static void out_vformat(Out& o, const char* format, va_list args) {
 
     /* conversion specifier */
     const char* str = buffer;
-    char strerror_buf[256];
     if (c == 's') {
       /* string */
       str = va_arg(args, const char*);
@@ -360,7 +359,7 @@ static void out_vformat(Out& o, const char* format, va_list args) {
       buffer[1] = 'x';
       format_integer(buffer + 2, sizeof(buffer) - 2, value, 'x');
     } else if (c == 'm') {
-      str = strerror_r(errno, strerror_buf, sizeof(strerror_buf));
+      strerror_r(errno, buffer, sizeof(buffer));
     } else if (c == 'd' || c == 'i' || c == 'o' || c == 'u' || c == 'x' || c == 'X') {
       /* integers - first read value from stack */
       uint64_t value;

tests/async_safe_test.cpp

@@ -16,6 +16,8 @@
 
 #include <gtest/gtest.h>
 
+#include <errno.h>
+
 #if defined(__BIONIC__)
 #include <async_safe/log.h>
 #endif // __BIONIC__
@@ -227,3 +229,19 @@ TEST(async_safe_log, buffer_overrun) {
   GTEST_SKIP() << "bionic-only test";
 #endif // __BIONIC__
 }
+
+// Verify that using %m is never cut off.
+TEST(async_safe_format_buffer, percent_m_fits_in_buffer) {
+#if defined(__BIONIC__)
+  for (int i = 0; i < 256; i++) {
+    errno = i;
+    char async_buf[256];
+    async_safe_format_buffer(async_buf, sizeof(async_buf), "%m");
+    char strerror_buf[1024];
+    strerror_r(errno, strerror_buf, sizeof(strerror_buf));
+    ASSERT_STREQ(strerror_buf, async_buf);
+  }
+#else   // __BIONIC__
+  GTEST_SKIP() << "bionic-only test";
+#endif  // __BIONIC__
+}