tracing: Fix filter string testing

The filter string testing uses strncpy_from_kernel/user_nofault() to
retrieve the string to test the filter against. The if() statement was
incorrect as it considered 0 as a fault, when it is only negative that it
faulted.

Running the following commands:

  # cd /sys/kernel/tracing
  # echo "filename.ustring ~ \"/proc*\"" > events/syscalls/sys_enter_openat/filter
  # echo 1 > events/syscalls/sys_enter_openat/enable
  # ls /proc/$$/maps
  # cat trace

Would produce nothing, but with the fix it will produce something like:

      ls-1192    [007] .....  8169.828333: sys_openat(dfd: ffffffffffffff9c, filename: 7efc18359904, flags: 80000, mode: 0)

Link: https://lore.kernel.org/all/CAEf4BzbVPQ=BjWztmEwBPRKHUwNfKBkS3kce-Rzka6zvbQeVpg@mail.gmail.com/
Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/20250417183003.505835fb@gandalf.local.home
Fixes: 77360f9bbc7e5 ("tracing: Add test for user space strings when filtering on string pointers")
Reported-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Reported-by: Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 0993dfc1c5c165d5f07ddb442ac66bb22f54f2ea..2048560264bb485da2df1181b92152e4d74d5a8f 100644 (file)
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -808,7 +808,7 @@ static __always_inline char *test_string(char *str)
        kstr = ubuf->buffer;
 
        /* For safety, do not trust the string pointer */
-       if (!strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE))
+       if (strncpy_from_kernel_nofault(kstr, str, USTRING_BUF_SIZE) < 0)
                return NULL;
        return kstr;
 }
@@ -827,7 +827,7 @@ static __always_inline char *test_ustring(char *str)
 
        /* user space address? */
        ustr = (char __user *)str;
-       if (!strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE))
+       if (strncpy_from_user_nofault(kstr, ustr, USTRING_BUF_SIZE) < 0)
                return NULL;
 
        return kstr;