On Thu, 17 Sep 2026 22:24:07 +0100
David Carlier <[email protected]> wrote:
> fprobe_fgraph_entry() reserves shadow stack space for every fprobe with
> an exit handler, but only fills it for those whose entry handler returns
> 0. fgraph_reserve_data() does not clear the area, so fprobe_return()
> parses the unused tail as headers left over from an earlier call, and an
> exit handler can run twice or despite its entry handler asking to skip
> it.
>
> Write a zero word after the last entry to terminate the walk. A zeroed
> slot does not decode to a NULL fprobe on the arches that encode the
> header into one unsigned long, since arch_decode_fprobe_header_fp() ORs
> in FPROBE_HEADER_MSB_PATTERN, so make read_fprobe_header() return NULL
> for a zeroed slot.
>
This fix looks good to me. Let me pick this.
Thanks!
> Fixes: e0a384434ae1 ("tracing: fprobe: do not zero out unused fgraph_data")
> Cc: [email protected]
> Suggested-by: Masami Hiramatsu (Google) <[email protected]>
> Signed-off-by: David Carlier <[email protected]>
> ---
> kernel/trace/fprobe.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index 1e9b00997ff2..9f2d98181779 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -171,6 +171,11 @@ static inline bool write_fprobe_header(unsigned long
> *stack,
> static inline void read_fprobe_header(unsigned long *stack,
> struct fprobe **fp, unsigned int
> *size_words)
> {
> + if (!*stack) {
> + *fp = NULL;
> + *size_words = 0;
> + return;
> + }
> *fp = arch_decode_fprobe_header_fp(*stack);
> *size_words = arch_decode_fprobe_header_size(*stack);
> }
> @@ -203,6 +208,12 @@ static inline void read_fprobe_header(unsigned long
> *stack,
> {
> struct __fprobe_header *fph = (struct __fprobe_header *)stack;
>
> + if (!*stack) {
> + *fp = NULL;
> + *size_words = 0;
> + return;
> + }
> +
> *fp = fph->fp;
> *size_words = fph->size_words;
> }
> @@ -635,6 +646,10 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent
> *trace, struct fgraph_ops
> }
> }
>
> + /* Terminate the list, fgraph_reserve_data() does not clear it. */
> + if (used && used < reserved_words)
> + fgraph_data[used] = 0;
> +
> /* If any exit_handler is set, data must be used. */
> return used != 0;
> }
> --
> 2.55.0
>
--
Masami Hiramatsu (Google) <[email protected]>
