Re: [PATCH bpf-next 5/9] libbpf: reduce reliance of attach_fns on sec_def internals

From: <hidden>
Date: 2021-09-17 17:28:05

On 09/16, Andrii Nakryiko wrote:

Move closer to not relying on bpf_sec_def internals that won't be part
of public API, when pluggable SEC() handlers will be allowed. Drop
pre-calculated prefix length, and in various helpers don't rely on this
prefix length availability. Also minimize reliance on knowing
bpf_sec_def's prefix for few places where section prefix shortcuts are
supported (e.g., tp vs tracepoint, raw_tp vs raw_tracepoint).

Given checking some string for having a given string-constant prefix is
such a common operation and so annoying to be done with pure C code, add
a small macro helper, str_has_pfx(), and reuse it throughout libbpf.c
where prefix comparison is performed. With __builtin_constant_p() it's
possible to have a convenient helper that checks some string for having
a given prefix, where prefix is either string literal (or compile-time
known string due to compiler optimization) or just a runtime string
pointer, which is quite convenient and saves a lot of typing and string
literal duplication.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
  tools/lib/bpf/libbpf.c          | 41 ++++++++++++++++++---------------
  tools/lib/bpf/libbpf_internal.h |  7 ++++++
  2 files changed, 30 insertions(+), 18 deletions(-)

quoted hunk ↗ jump to hunk

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8ab2edbf7a3b..52c398fae0af 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c

@@ -226,7 +226,6 @@ typedef struct bpf_link *(*attach_fn_t)(const struct

bpf_program *prog, long coo

quoted hunk ↗ jump to hunk

  struct bpf_sec_def {
  	const char *sec;
-	size_t len;
  	enum bpf_prog_type prog_type;
  	enum bpf_attach_type expected_attach_type;
  	bool is_exp_attach_type_optional;

@@ -1671,7 +1670,7 @@ static int bpf_object__process_kconfig_line(struct

bpf_object *obj,
  	void *ext_val;
  	__u64 num;

-	if (strncmp(buf, "CONFIG_", 7))
+	if (!str_has_pfx(buf, "CONFIG_"))
  		return 0;

quoted hunk ↗ jump to hunk

  	sep = strchr(buf, '=');

@@ -2919,7 +2918,7 @@ static Elf_Data *elf_sec_data(const struct

bpf_object *obj, Elf_Scn *scn)
  static bool is_sec_name_dwarf(const char *name)
  {
  	/* approximation, but the actual list is too long */
-	return strncmp(name, ".debug_", sizeof(".debug_") - 1) == 0;
+	return str_has_pfx(name, ".debug_");
  }

quoted hunk ↗ jump to hunk

  static bool ignore_elf_section(GElf_Shdr *hdr, const char *name)

@@ -2941,7 +2940,7 @@ static bool ignore_elf_section(GElf_Shdr *hdr,

const char *name)
  	if (is_sec_name_dwarf(name))
  		return true;

quoted hunk ↗ jump to hunk

-	if (strncmp(name, ".rel", sizeof(".rel") - 1) == 0) {
+	if (str_has_pfx(name, ".rel")) {
  		name += sizeof(".rel") - 1;
  		/* DWARF section relocations */
  		if (is_sec_name_dwarf(name))

@@ -6890,8 +6889,7 @@ static int bpf_object__resolve_externs(struct

bpf_object *obj,
  			if (err)
  				return err;
  			pr_debug("extern (kcfg) %s=0x%x\n", ext->name, kver);
-		} else if (ext->type == EXT_KCFG &&
-			   strncmp(ext->name, "CONFIG_", 7) == 0) {
+		} else if (ext->type == EXT_KCFG && str_has_pfx(ext->name, "CONFIG_"))  
{
  			need_config = true;
  		} else if (ext->type == EXT_KSYM) {
  			if (ext->ksym.type_id)

@@ -7955,7 +7953,6 @@ void bpf_program__set_expected_attach_type(struct

bpf_program *prog,
  			  attachable, attach_btf)			    \
  	{								    \
  		.sec = string,						    \
-		.len = sizeof(string) - 1,				    \
  		.prog_type = ptype,					    \
  		.expected_attach_type = eatype,				    \
  		.is_exp_attach_type_optional = eatype_optional,		    \

@@ -7986,7 +7983,6 @@ void bpf_program__set_expected_attach_type(struct

bpf_program *prog,

quoted hunk ↗ jump to hunk

  #define SEC_DEF(sec_pfx, ptype, ...) {					    \
  	.sec = sec_pfx,							    \
-	.len = sizeof(sec_pfx) - 1,					    \
  	.prog_type = BPF_PROG_TYPE_##ptype,				    \
  	.preload_fn = libbpf_preload_prog,				    \
  	__VA_ARGS__							    \

@@ -8160,10 +8156,8 @@ static const struct bpf_sec_def

*find_sec_def(const char *sec_name)
  	int i, n = ARRAY_SIZE(section_defs);

quoted hunk ↗ jump to hunk

  	for (i = 0; i < n; i++) {
-		if (strncmp(sec_name,
-			    section_defs[i].sec, section_defs[i].len))
-			continue;
-		return &section_defs[i];
+		if (str_has_pfx(sec_name, section_defs[i].sec))
+			return &section_defs[i];
  	}
  	return NULL;
  }

@@ -8517,7 +8511,7 @@ static int libbpf_find_attach_btf_id(struct

bpf_program *prog, int *btf_obj_fd,
  			prog->sec_name);
  		return -ESRCH;
  	}
-	attach_name = prog->sec_name + prog->sec_def->len;
+	attach_name = prog->sec_name + strlen(prog->sec_def->sec);

quoted hunk ↗ jump to hunk

  	/* BPF program's BTF ID */
  	if (attach_prog_fd) {

@@ -9454,8 +9448,11 @@ static struct bpf_link *attach_kprobe(const struct

bpf_program *prog, long cooki
  	char *func;
  	int n, err;

-	func_name = prog->sec_name + prog->sec_def->len;
-	opts.retprobe = strcmp(prog->sec_def->sec, "kretprobe/") == 0;
+	opts.retprobe = str_has_pfx(prog->sec_name, "kretprobe/");
+	if (opts.retprobe)
+		func_name = prog->sec_name + sizeof("kretprobe/") - 1;
+	else
+		func_name = prog->sec_name + sizeof("kprobe/") - 1;

quoted hunk ↗ jump to hunk

  	n = sscanf(func_name, "%m[a-zA-Z0-9_.]+%li", &func, &offset);
  	if (n < 1) {

@@ -9627,8 +9624,11 @@ static struct bpf_link *attach_tp(const struct

bpf_program *prog, long cookie)
  	if (!sec_name)
  		return libbpf_err_ptr(-ENOMEM);

quoted hunk ↗ jump to hunk

-	/* extract "tp/<category>/<name>" */
-	tp_cat = sec_name + prog->sec_def->len;
+	/* extract "tp/<category>/<name>" or "tracepoint/<category>/<name>" */
+	if (str_has_pfx(prog->sec_name, "tp/"))
+		tp_cat = sec_name + sizeof("tp/") - 1;
+	else
+		tp_cat = sec_name + sizeof("tracepoint/") - 1;
  	tp_name = strchr(tp_cat, '/');
  	if (!tp_name) {
  		free(sec_name);

@@ -9674,7 +9674,12 @@ struct bpf_link

*bpf_program__attach_raw_tracepoint(const struct bpf_program *pr

  static struct bpf_link *attach_raw_tp(const struct bpf_program *prog,  
long cookie)
  {
-	const char *tp_name = prog->sec_name + prog->sec_def->len;
+	const char *tp_name;
+
+	if (str_has_pfx(prog->sec_name, "raw_tp/"))
+		tp_name = prog->sec_name + sizeof("raw_tp/") - 1;
+	else
+		tp_name = prog->sec_name + sizeof("raw_tracepoint/") - 1;

quoted hunk ↗ jump to hunk

  	return bpf_program__attach_raw_tracepoint(prog, tp_name);
  }

diff --git a/tools/lib/bpf/libbpf_internal.h

b/tools/lib/bpf/libbpf_internal.h
index ceb0c98979bc..ec79400517d4 100644

--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h

@@ -89,6 +89,13 @@
  	(offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD))
  #endif

+/* Check whether a string `str` has prefix `pfx`, regardless if `pfx` is
+ * a string literal known at compilation time or char * pointer known  
only at
+ * runtime.
+ */
+#define str_has_pfx(str, pfx) \
+	(strncmp(str, pfx, __builtin_constant_p(pfx) ? sizeof(pfx) - 1 :  
strlen(pfx)) == 0)
+

Nit: maybe 'starts_with'? c++ has that in stdlib and iirc python also
has either starts_with or statswith.

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