Re: [PATCH 1/1] perf trace: Use heuristic when deciding if a syscall tracepoint "const char *" field is really a string
From: Ian Rogers
Date: Thu Aug 17 2023 - 14:47:49 EST
On Thu, Aug 17, 2023 at 11:23 AM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> 'perf trace' tries to find BPF progs associated with a syscall that have
> a signature that is similar to syscalls without one to try and reuse,
> so, for instance, the 'open' signature can be reused with many other
> syscalls that have as its first arg a string.
>
> It uses the tracefs events format file for finding a signature that can
> be reused, but then comes the "write" syscall with its second argument
> as a "const char *":
>
> # cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_write/format
> name: sys_enter_write
> ID: 746
> format:
> field:unsigned short common_type; offset:0; size:2; signed:0;
> field:unsigned char common_flags; offset:2; size:1; signed:0;
> field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
> field:int common_pid; offset:4; size:4; signed:1;
>
> field:int __syscall_nr; offset:8; size:4; signed:1;
> field:unsigned int fd; offset:16; size:8; signed:0;
> field:const char * buf; offset:24; size:8; signed:0;
> field:size_t count; offset:32; size:8; signed:0;
>
> print fmt: "fd: 0x%08lx, buf: 0x%08lx, count: 0x%08lx", ((unsigned long)(REC->fd)), ((unsigned long)(REC->buf)), ((unsigned long)(REC->count))
> #
>
> Which isn't a string (the man page for glibc has buf as "void *"), so we
> have to use the name of the argument as an heuristic, to consider a
> string just args that are "const char *" and that have in its name the
> "path", "file", etc substrings.
>
> With that now it reuses:
>
> [root@quaco ~]# perf trace -v --max-events=1 |& grep Reus
> Reusing "open" BPF sys_enter augmenter for "stat"
> Reusing "open" BPF sys_enter augmenter for "lstat"
> Reusing "open" BPF sys_enter augmenter for "access"
> Reusing "connect" BPF sys_enter augmenter for "accept"
> Reusing "sendto" BPF sys_enter augmenter for "recvfrom"
> Reusing "connect" BPF sys_enter augmenter for "bind"
> Reusing "connect" BPF sys_enter augmenter for "getsockname"
> Reusing "connect" BPF sys_enter augmenter for "getpeername"
> Reusing "open" BPF sys_enter augmenter for "execve"
> Reusing "open" BPF sys_enter augmenter for "truncate"
> Reusing "open" BPF sys_enter augmenter for "chdir"
> Reusing "open" BPF sys_enter augmenter for "mkdir"
> Reusing "open" BPF sys_enter augmenter for "rmdir"
> Reusing "open" BPF sys_enter augmenter for "creat"
> Reusing "open" BPF sys_enter augmenter for "link"
> Reusing "open" BPF sys_enter augmenter for "unlink"
> Reusing "open" BPF sys_enter augmenter for "symlink"
> Reusing "open" BPF sys_enter augmenter for "readlink"
> Reusing "open" BPF sys_enter augmenter for "chmod"
> Reusing "open" BPF sys_enter augmenter for "chown"
> Reusing "open" BPF sys_enter augmenter for "lchown"
> Reusing "open" BPF sys_enter augmenter for "mknod"
> Reusing "open" BPF sys_enter augmenter for "statfs"
> Reusing "open" BPF sys_enter augmenter for "pivot_root"
> Reusing "open" BPF sys_enter augmenter for "chroot"
> Reusing "open" BPF sys_enter augmenter for "acct"
> Reusing "open" BPF sys_enter augmenter for "swapon"
> Reusing "open" BPF sys_enter augmenter for "swapoff"
> Reusing "open" BPF sys_enter augmenter for "delete_module"
> Reusing "open" BPF sys_enter augmenter for "setxattr"
> Reusing "open" BPF sys_enter augmenter for "lsetxattr"
> Reusing "openat" BPF sys_enter augmenter for "fsetxattr"
> Reusing "open" BPF sys_enter augmenter for "getxattr"
> Reusing "open" BPF sys_enter augmenter for "lgetxattr"
> Reusing "openat" BPF sys_enter augmenter for "fgetxattr"
> Reusing "open" BPF sys_enter augmenter for "listxattr"
> Reusing "open" BPF sys_enter augmenter for "llistxattr"
> Reusing "open" BPF sys_enter augmenter for "removexattr"
> Reusing "open" BPF sys_enter augmenter for "lremovexattr"
> Reusing "fsetxattr" BPF sys_enter augmenter for "fremovexattr"
> Reusing "open" BPF sys_enter augmenter for "mq_open"
> Reusing "open" BPF sys_enter augmenter for "mq_unlink"
> Reusing "fsetxattr" BPF sys_enter augmenter for "add_key"
> Reusing "fremovexattr" BPF sys_enter augmenter for "request_key"
> Reusing "fremovexattr" BPF sys_enter augmenter for "inotify_add_watch"
> Reusing "fremovexattr" BPF sys_enter augmenter for "mkdirat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "mknodat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "fchownat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "futimesat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "newfstatat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "unlinkat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "linkat"
> Reusing "open" BPF sys_enter augmenter for "symlinkat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "readlinkat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "fchmodat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "faccessat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "utimensat"
> Reusing "connect" BPF sys_enter augmenter for "accept4"
> Reusing "fremovexattr" BPF sys_enter augmenter for "name_to_handle_at"
> Reusing "fremovexattr" BPF sys_enter augmenter for "renameat2"
> Reusing "open" BPF sys_enter augmenter for "memfd_create"
> Reusing "fremovexattr" BPF sys_enter augmenter for "execveat"
> Reusing "fremovexattr" BPF sys_enter augmenter for "statx"
> [root@quaco ~]#
>
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Alan Maguire <alan.maguire@xxxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Reviewed-by: Ian Rogers <irogers@xxxxxxxxxx>
Thanks,
Ian
> ---
> tools/perf/builtin-trace.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 3964cf44cdbcb3e8..e541d0e2777ab935 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -3398,6 +3398,19 @@ static struct bpf_program *trace__find_usable_bpf_prog_entry(struct trace *trace
> if (strcmp(field->type, candidate_field->type))
> goto next_candidate;
>
> + /*
> + * This is limited in the BPF program but sys_write
> + * uses "const char *" for its "buf" arg so we need to
> + * use some heuristic that is kinda future proof...
> + */
> + if (strcmp(field->type, "const char *") == 0 &&
> + !(strstr(field->name, "name") ||
> + strstr(field->name, "path") ||
> + strstr(field->name, "file") ||
> + strstr(field->name, "root") ||
> + strstr(field->name, "description")))
> + goto next_candidate;
> +
> is_candidate = true;
> }
>
> --
> 2.41.0
>