Subject: [RFC bpf-next 3/4] bpf: Resolve symbols with kallsyms_lookup_names for kprobe multi link

From: Jiri Olsa <[email-masked]>
Date: Thu, 7 Apr 2022 14:52:23 +0200

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% )

Signed-off-by: Jiri Olsa <[email-masked]>
---
 kernel/trace/bpf_trace.c | 123 +++++++++++++++++++++++----------------
 1 file changed, 73 insertions(+), 50 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b26f3da943de..2602957225ba 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -2226,6 +2226,72 @@ 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, void __user *usyms, u32 cnt)
+{
+	const char __user **usyms_copy = NULL;
+	const char **syms = NULL;
+	char *buf = NULL, *p;
+	int err = -EFAULT;
+	unsigned int i;
+	size_t size;
+
+	size = cnt * sizeof(*usyms_copy);
+
+	usyms_copy = kvmalloc(size, GFP_KERNEL);
+	if (!usyms_copy)
+		return -ENOMEM;
+
+	if (copy_from_user(usyms_copy, usyms, size))
+		goto error;
+
+	err = -ENOMEM;
+	syms = kvmalloc(size, GFP_KERNEL);
+	if (!syms)
+		goto error;
+
+	/* TODO this potentially allocates lot of memory (~6MB in my tests
+	 * with attaching ~40k functions). I haven't seen this to fail yet,
+	 * but it could be changed to allocate memory gradually if needed.
+	 */
+	size = cnt * KSYM_NAME_LEN;
+	buf = kvmalloc(size, GFP_KERNEL);
+	if (!buf)
+		goto error;
+
+	for (p = buf, i = 0; i < cnt; i++) {
+		err = strncpy_from_user(p, usyms_copy[i], KSYM_NAME_LEN);
+		if (err == KSYM_NAME_LEN)
+			err = -E2BIG;
+		if (err < 0)
+			goto error;
+		syms[i] = p;
+		p += err + 1;
+	}
+
+	err = 0;
+	us->syms = syms;
+	us->buf = buf;
+
+error:
+	kvfree(usyms_copy);
+	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;
@@ -2346,55 +2412,6 @@ 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)
-{
-	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;
-
-	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;
-}
-
 int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 {
 	struct bpf_kprobe_multi_link *link = NULL;
@@ -2438,7 +2455,13 @@ 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;
+		err = kallsyms_lookup_names(us.syms, cnt, addrs);
+		free_user_syms(&us);
 		if (err)
 			goto error;
 	}
-- 
2.35.1