libbpf: Skip well-known ELF sections when iterating ELF

Skip and don't log ELF sections that libbpf knows about and ignores during ELF
processing. This allows to not unnecessarily log details about those ELF
sections and cleans up libbpf debug log. Ignored sections include DWARF data,
string table, empty .text section and few special (e.g., .llvm_addrsig)
useless sections.

With such ELF sections out of the way, log unrecognized ELF sections at
pr_info level to increase visibility.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200820231250.1293069-5-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -2672,6 +2672,46 @@ static Elf_Data *elf_sec_data(const struct bpf_object *obj, Elf_Scn *scn)
 	return data;
 }
 
+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;
+}
+
+static bool ignore_elf_section(GElf_Shdr *hdr, const char *name)
+{
+	/* no special handling of .strtab */
+	if (hdr->sh_type == SHT_STRTAB)
+		return true;
+
+	/* ignore .llvm_addrsig section as well */
+	if (hdr->sh_type == 0x6FFF4C03 /* SHT_LLVM_ADDRSIG */)
+		return true;
+
+	/* no subprograms will lead to an empty .text section, ignore it */
+	if (hdr->sh_type == SHT_PROGBITS && hdr->sh_size == 0 &&
+	    strcmp(name, ".text") == 0)
+		return true;
+
+	/* DWARF sections */
+	if (is_sec_name_dwarf(name))
+		return true;
+
+	if (strncmp(name, ".rel", sizeof(".rel") - 1) == 0) {
+		name += sizeof(".rel") - 1;
+		/* DWARF section relocations */
+		if (is_sec_name_dwarf(name))
+			return true;
+
+		/* .BTF and .BTF.ext don't need relocations */
+		if (strcmp(name, BTF_ELF_SEC) == 0 ||
+		    strcmp(name, BTF_EXT_ELF_SEC) == 0)
+			return true;
+	}
+
+	return false;
+}
+
 static int bpf_object__elf_collect(struct bpf_object *obj)
 {
 	Elf *elf = obj->efile.elf;
@@ -2694,6 +2734,9 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		if (!name)
 			return -LIBBPF_ERRNO__FORMAT;
 
+		if (ignore_elf_section(&sh, name))
+			continue;
+
 		data = elf_sec_data(obj, scn);
 		if (!data)
 			return -LIBBPF_ERRNO__FORMAT;
@@ -2746,8 +2789,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 				obj->efile.st_ops_data = data;
 				obj->efile.st_ops_shndx = idx;
 			} else {
-				pr_debug("elf: skipping unrecognized data section(%d) %s\n",
-					 idx, name);
+				pr_info("elf: skipping unrecognized data section(%d) %s\n",
+					idx, name);
 			}
 		} else if (sh.sh_type == SHT_REL) {
 			int nr_sects = obj->efile.nr_reloc_sects;
@@ -2758,9 +2801,9 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 			if (!section_have_execinstr(obj, sec) &&
 			    strcmp(name, ".rel" STRUCT_OPS_SEC) &&
 			    strcmp(name, ".rel" MAPS_ELF_SEC)) {
-				pr_debug("elf: skipping relo section(%d) %s for section(%d) %s\n",
-					 idx, name, sec,
-					 elf_sec_name(obj, elf_sec_by_idx(obj, sec)) ?: "<?>");
+				pr_info("elf: skipping relo section(%d) %s for section(%d) %s\n",
+					idx, name, sec,
+					elf_sec_name(obj, elf_sec_by_idx(obj, sec)) ?: "<?>");
 				continue;
 			}
 
@@ -2778,7 +2821,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 			obj->efile.bss = data;
 			obj->efile.bss_shndx = idx;
 		} else {
-			pr_debug("elf: skipping section(%d) %s\n", idx, name);
+			pr_info("elf: skipping section(%d) %s (size %zu)\n", idx, name, sh.sh_size);
 		}
 	}