libbpf: Introduce elf_find_func_offset_from_file() function

This change splits the elf_find_func_offset() function in two:
elf_find_func_offset(), which now accepts an already opened Elf object
instead of a path to a file that is to be opened, as well as
elf_find_func_offset_from_file(), which opens a binary based on a
path and then invokes elf_find_func_offset() on the Elf object. Having
this split in responsibilities will allow us to call
elf_find_func_offset() from other code paths on Elf objects that did not
necessarily come from a file on disk.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230301212308.1839139-3-deso@posteo.net

tools/lib/bpf/libbpf.c

@@ -10530,32 +10530,19 @@ static Elf_Scn *elf_find_next_scn_by_type(Elf *elf, int sh_type, Elf_Scn *scn)
 	return NULL;
 }
 
-/* Find offset of function name in object specified by path.  "name" matches
- * symbol name or name@@LIB for library functions.
+/* Find offset of function name in the provided ELF object. "binary_path" is
+ * the path to the ELF binary represented by "elf", and only used for error
+ * reporting matters. "name" matches symbol name or name@@LIB for library
+ * functions.
  */
-static long elf_find_func_offset(const char *binary_path, const char *name)
+static long elf_find_func_offset(Elf *elf, const char *binary_path, const char *name)
 {
-	int fd, i, sh_types[2] = { SHT_DYNSYM, SHT_SYMTAB };
+	int i, sh_types[2] = { SHT_DYNSYM, SHT_SYMTAB };
 	bool is_shared_lib, is_name_qualified;
-	char errmsg[STRERR_BUFSIZE];
 	long ret = -ENOENT;
 	size_t name_len;
 	GElf_Ehdr ehdr;
-	Elf *elf;
 
-	fd = open(binary_path, O_RDONLY | O_CLOEXEC);
-	if (fd < 0) {
-		ret = -errno;
-		pr_warn("failed to open %s: %s\n", binary_path,
-			libbpf_strerror_r(ret, errmsg, sizeof(errmsg)));
-		return ret;
-	}
-	elf = elf_begin(fd, ELF_C_READ_MMAP, NULL);
-	if (!elf) {
-		pr_warn("elf: could not read elf from %s: %s\n", binary_path, elf_errmsg(-1));
-		close(fd);
-		return -LIBBPF_ERRNO__FORMAT;
-	}
 	if (!gelf_getehdr(elf, &ehdr)) {
 		pr_warn("elf: failed to get ehdr from %s: %s\n", binary_path, elf_errmsg(-1));
 		ret = -LIBBPF_ERRNO__FORMAT;
@@ -10568,7 +10555,7 @@ static long elf_find_func_offset(const char *binary_path, const char *name)
 	/* Does name specify "@@LIB"? */
 	is_name_qualified = strstr(name, "@@") != NULL;
 
-	/* Search SHT_DYNSYM, SHT_SYMTAB for symbol.  This search order is used because if
+	/* Search SHT_DYNSYM, SHT_SYMTAB for symbol. This search order is used because if
 	 * a binary is stripped, it may only have SHT_DYNSYM, and a fully-statically
 	 * linked binary may not have SHT_DYMSYM, so absence of a section should not be
 	 * reported as a warning/error.
@@ -10681,6 +10668,34 @@ static long elf_find_func_offset(const char *binary_path, const char *name)
 		}
 	}
 out:
+	return ret;
+}
+
+/* Find offset of function name in ELF object specified by path. "name" matches
+ * symbol name or name@@LIB for library functions.
+ */
+static long elf_find_func_offset_from_file(const char *binary_path, const char *name)
+{
+	char errmsg[STRERR_BUFSIZE];
+	long ret = -ENOENT;
+	Elf *elf;
+	int fd;
+
+	fd = open(binary_path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		ret = -errno;
+		pr_warn("failed to open %s: %s\n", binary_path,
+			libbpf_strerror_r(ret, errmsg, sizeof(errmsg)));
+		return ret;
+	}
+	elf = elf_begin(fd, ELF_C_READ_MMAP, NULL);
+	if (!elf) {
+		pr_warn("elf: could not read elf from %s: %s\n", binary_path, elf_errmsg(-1));
+		close(fd);
+		return -LIBBPF_ERRNO__FORMAT;
+	}
+
+	ret = elf_find_func_offset(elf, binary_path, name);
 	elf_end(elf);
 	close(fd);
 	return ret;
@@ -10804,7 +10819,7 @@ bpf_program__attach_uprobe_opts(const struct bpf_program *prog, pid_t pid,
 	if (func_name) {
 		long sym_off;
 
-		sym_off = elf_find_func_offset(binary_path, func_name);
+		sym_off = elf_find_func_offset_from_file(binary_path, func_name);
 		if (sym_off < 0)
 			return libbpf_err_ptr(sym_off);
 		func_offset += sym_off;