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libbpf: Add support to detect nop,nop5 instructions combo for usdt probe

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Adding support to detect nop,nop5 instructions combo for usdt probe
by checking on probe's following nop5 instruction.

When the nop,nop5 combo is detected together with uprobe syscall,
we can place the probe on top of nop5 and get it optimized.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260224103915.1369690-3-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Jiri Olsa
2026-02-24 11:39:12 +01:00
committed by Andrii Nakryiko
parent 8cbfbe1415
commit 67cce78af9
1 changed files with 43 additions and 4 deletions
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47
src/usdt.c
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@@ -262,6 +262,7 @@ struct usdt_manager {
bool has_bpf_cookie;
bool has_sema_refcnt;
bool has_uprobe_multi;
bool has_uprobe_syscall;
};
struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
@@ -301,6 +302,13 @@ struct usdt_manager *usdt_manager_new(struct bpf_object *obj)
* usdt probes.
*/
man->has_uprobe_multi = kernel_supports(obj, FEAT_UPROBE_MULTI_LINK);
/*
* Detect kernel support for uprobe() syscall, it's presence means we can
* take advantage of faster nop5 uprobe handling.
* Added in: 56101b69c919 ("uprobes/x86: Add uprobe syscall to speed up uprobe")
*/
man->has_uprobe_syscall = kernel_supports(obj, FEAT_UPROBE_SYSCALL);
return man;
}
@@ -585,13 +593,34 @@ static int parse_usdt_note(GElf_Nhdr *nhdr, const char *data, size_t name_off,
static int parse_usdt_spec(struct usdt_spec *spec, const struct usdt_note *note, __u64 usdt_cookie);
static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *path, pid_t pid,
const char *usdt_provider, const char *usdt_name, __u64 usdt_cookie,
struct usdt_target **out_targets, size_t *out_target_cnt)
#if defined(__x86_64__)
static bool has_nop_combo(int fd, long off)
{
unsigned char nop_combo[6] = {
0x90, 0x0f, 0x1f, 0x44, 0x00, 0x00 /* nop,nop5 */
};
unsigned char buf[6];
if (pread(fd, buf, 6, off) != 6)
return false;
return memcmp(buf, nop_combo, 6) == 0;
}
#else
static bool has_nop_combo(int fd, long off)
{
return false;
}
#endif
static int collect_usdt_targets(struct usdt_manager *man, struct elf_fd *elf_fd, const char *path,
pid_t pid, const char *usdt_provider, const char *usdt_name,
__u64 usdt_cookie, struct usdt_target **out_targets,
size_t *out_target_cnt)
{
size_t off, name_off, desc_off, seg_cnt = 0, vma_seg_cnt = 0, target_cnt = 0;
struct elf_seg *segs = NULL, *vma_segs = NULL;
struct usdt_target *targets = NULL, *target;
Elf *elf = elf_fd->elf;
long base_addr = 0;
Elf_Scn *notes_scn, *base_scn;
GElf_Shdr base_shdr, notes_shdr;
@@ -784,6 +813,16 @@ static int collect_usdt_targets(struct usdt_manager *man, Elf *elf, const char *
target = &targets[target_cnt];
memset(target, 0, sizeof(*target));
/*
* We have uprobe syscall and usdt with nop,nop5 instructions combo,
* so we can place the uprobe directly on nop5 (+1) and get this probe
* optimized.
*/
if (man->has_uprobe_syscall && has_nop_combo(elf_fd->fd, usdt_rel_ip)) {
usdt_abs_ip++;
usdt_rel_ip++;
}
target->abs_ip = usdt_abs_ip;
target->rel_ip = usdt_rel_ip;
target->sema_off = usdt_sema_off;
@@ -998,7 +1037,7 @@ struct bpf_link *usdt_manager_attach_usdt(struct usdt_manager *man, const struct
/* discover USDT in given binary, optionally limiting
* activations to a given PID, if pid > 0
*/
err = collect_usdt_targets(man, elf_fd.elf, path, pid, usdt_provider, usdt_name,
err = collect_usdt_targets(man, &elf_fd, path, pid, usdt_provider, usdt_name,
usdt_cookie, &targets, &target_cnt);
if (err <= 0) {
err = (err == 0) ? -ENOENT : err;