[PATCHv4 bpf-next 4/5] bpf: Resolve symbols with ftrace_lookup_symbols for kprobe multi link

Jiri Olsa
2022-04-28T20:13:13Z

Using kallsyms_lookup_names function to speed up symbols lookup in
kprobe multi link attachment and replacing with it the current
kprobe_multi_resolve_syms function.

This speeds up bpftrace kprobe attachment:

# perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* { } i:ms:1 { exit(); }'
...
6.5681 +- 0.0225 seconds time elapsed ( +- 0.34% )

After:

# perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* { } i:ms:1 { exit(); }'
...
0.5661 +- 0.0275 seconds time elapsed ( +- 4.85% )

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
kernel/trace/bpf_trace.c | 112 +++++++++++++++++++++++----------------
1 file changed, 66 insertions(+), 46 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index f15b826f9899..7fd11c17558d 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2229,6 +2229,59 @@ struct bpf_kprobe_multi_run_ctx {
unsigned long entry_ip;
};

+struct user_syms {
+ const char **syms;
+ char *buf;
+};
+
+static int copy_user_syms(struct user_syms *us, unsigned long __user *usyms, u32 cnt)
+{
+ unsigned long __user usymbol;
+ const char **syms = NULL;
+ char *buf = NULL, *p;
+ int err = -ENOMEM;
+ unsigned int i;
+
+ syms = kvmalloc(cnt * sizeof(*syms), GFP_KERNEL);
+ if (!syms)
+ goto error;
+
+ buf = kvmalloc(cnt * KSYM_NAME_LEN, GFP_KERNEL);
+ if (!buf)
+ goto error;
+
+ for (p = buf, i = 0; i < cnt; i++) {
+ if (__get_user(usymbol, usyms + i)) {
+ err = -EFAULT;
+ goto error;
+ }
+ err = strncpy_from_user(p, (const char __user *) usymbol, KSYM_NAME_LEN);
+ if (err == KSYM_NAME_LEN)
+ err = -E2BIG;
+ if (err < 0)
+ goto error;
+ syms[i] = p;
+ p += err + 1;
+ }
+
+ us->syms = syms;
+ us->buf = buf;
+ return 0;
+
+error:
+ if (err) {
+ kvfree(syms);
+ kvfree(buf);
+ }
+ return err;
+}
+
+static void free_user_syms(struct user_syms *us)
+{
+ kvfree(us->syms);
+ kvfree(us->buf);
+}
+
static void bpf_kprobe_multi_link_release(struct bpf_link *link)
{
struct bpf_kprobe_multi_link *kmulti_link;
@@ -2349,53 +2402,12 @@ kprobe_multi_link_handler(struct fprobe *fp, unsigned long entry_ip,
kprobe_multi_link_prog_run(link, entry_ip, regs);
}

-static int
-kprobe_multi_resolve_syms(const void __user *usyms, u32 cnt,
- unsigned long *addrs)
+static int symbols_cmp(const void *a, const void *b)
{
- unsigned long addr, size;
- const char __user **syms;
- int err = -ENOMEM;
- unsigned int i;
- char *func;
-
- size = cnt * sizeof(*syms);
- syms = kvzalloc(size, GFP_KERNEL);
- if (!syms)
- return -ENOMEM;
+ const char **str_a = (const char **) a;
+ const char **str_b = (const char **) b;

- func = kmalloc(KSYM_NAME_LEN, GFP_KERNEL);
- if (!func)
- goto error;
-
- if (copy_from_user(syms, usyms, size)) {
- err = -EFAULT;
- goto error;
- }
-
- for (i = 0; i < cnt; i++) {
- err = strncpy_from_user(func, syms[i], KSYM_NAME_LEN);
- if (err == KSYM_NAME_LEN)
- err = -E2BIG;
- if (err < 0)
- goto error;
- err = -EINVAL;
- addr = kallsyms_lookup_name(func);
- if (!addr)
- goto error;
- if (!kallsyms_lookup_size_offset(addr, &size, NULL))
- goto error;
- addr = ftrace_location_range(addr, addr + size - 1);
- if (!addr)
- goto error;
- addrs[i] = addr;
- }
-
- err = 0;
-error:
- kvfree(syms);
- kfree(func);
- return err;
+ return strcmp(*str_a, *str_b);
}

int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
@@ -2441,7 +2453,15 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
goto error;
}
} else {
- err = kprobe_multi_resolve_syms(usyms, cnt, addrs);
+ struct user_syms us;
+
+ err = copy_user_syms(&us, usyms, cnt);
+ if (err)
+ goto error;
+
+ sort(us.syms, cnt, sizeof(*us.syms), symbols_cmp, NULL);
+ err = ftrace_lookup_symbols(us.syms, cnt, addrs);
+ free_user_syms(&us);
if (err)
goto error;
}
--
2.35.1