Re: [Qemu-devel] [PATCH 1/2] trace: handle tracefs path truncation

[Liam Merwick]

On 21/03/2019 17:08, Stefan Hajnoczi wrote:

If the tracefs mountpoint has a very long path we may exceed PATH_MAX.
This is a system misconfiguration and the user must resolve it so that
applications can perform path-based system calls successfully.

This issue does not occur on real-world systems since tracefs is mounted
on /sys/kernel/debug/tracing/, but the compiler is smart enough to
foresee the possibility and warn about the unchecked snprintf(3) return
value.  This patch fixes the compiler warning.

Reported-by: Markus Armbruster <arm...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>

Reviewed-by: Liam Merwick <liam.merw...@oracle.com>

---
  trace/ftrace.c | 12 ++++++++++--
  1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/trace/ftrace.c b/trace/ftrace.c
index 61692a8682..9749543d9b 100644
--- a/trace/ftrace.c
+++ b/trace/ftrace.c
@@ -53,7 +53,11 @@ bool ftrace_init(void)
      }

if (tracefs_found) {

-        snprintf(path, PATH_MAX, "%s%s/tracing_on", mount_point, subdir);
+        if (snprintf(path, PATH_MAX, "%s%s/tracing_on", mount_point, subdir)
+                >= sizeof(path)) {
+            fprintf(stderr, "Using tracefs mountpoint would exceed 
PATH_MAX\n");
+            return false;
+        }
          trace_fd = open(path, O_WRONLY);
          if (trace_fd < 0) {
              if (errno == EACCES) {
@@ -72,7 +76,11 @@ bool ftrace_init(void)
              }
              close(trace_fd);
          }
-        snprintf(path, PATH_MAX, "%s%s/trace_marker", mount_point, subdir);
+        if (snprintf(path, PATH_MAX, "%s%s/trace_marker", mount_point, subdir)
+                >= sizeof(path)) {
+            fprintf(stderr, "Using tracefs mountpoint would exceed 
PATH_MAX\n");
+            return false;
+        }
          trace_marker_fd = open(path, O_WRONLY);
          if (trace_marker_fd < 0) {
              perror("Could not open ftrace 'trace_marker' file");