Re: [PATCH 1/2] ftrace: Drop duplicate mcount locations

Steven Rostedt
2022-04-27T13:47:08Z

On Wed, 27 Apr 2022 15:01:21 +0530
"Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

> In the absence of section symbols [1], objtool (today) and recordmcount
> (with a subsequent patch) generate __mcount_loc relocation records with
> weak symbols as the base. This works fine as long as those weak symbols
> are not overridden, but if they are, these can result in duplicate
> entries in the final vmlinux mcount location table. This will cause
> ftrace to fail when trying to patch the same location twice. Fix this by
> dropping duplicate locations during ftrace init.
>
> [1] https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=d1bcae833b32f1
>
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
> kernel/trace/ftrace.c | 13 ++++++++++++-
> 1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 4f1d2f5e726341..8bc4f282bb3ff4 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -6496,7 +6496,7 @@ static int ftrace_process_locs(struct module *mod,
> struct dyn_ftrace *rec;
> unsigned long count;
> unsigned long *p;
> - unsigned long addr;
> + unsigned long addr, prev_addr = 0;
> unsigned long flags = 0; /* Shut up gcc */
> int ret = -ENOMEM;
>
> @@ -6550,6 +6550,16 @@ static int ftrace_process_locs(struct module *mod,
> while (p < end) {
> unsigned long end_offset;
> addr = ftrace_call_adjust(*p++);
> +
> + /*
> + * Drop duplicate entries, which can happen when weak
> + * functions are overridden, and __mcount_loc relocation
> + * records were generated against function names due to
> + * absence of non-weak section symbols
> + */
> + if (addr == prev_addr)
> + addr = 0;

Please don't use the side effect of addr == 0 causing the loop to continue
for this logic. The two are not related. Simply call continue.

if (addr == prev_addr)
continue;

-- Steve

> +
> /*
> * Some architecture linkers will pad between
> * the different mcount_loc sections of different
> @@ -6569,6 +6579,7 @@ static int ftrace_process_locs(struct module *mod,
>
> rec = &pg->records[pg->index++];
> rec->ip = addr;
> + prev_addr = addr;
> }
>
> /* We should have used all pages */