libbpf: Extend linker API to support in-memory ELF files

The new_fd and add_fd functions correspond to the original new and
add_file functions, but accept an FD instead of a file name. This
gives API consumers the option of using anonymous files/memfds to
avoid writing ELFs to disk.

This new API will be useful for performing linking as part of
bpftrace's JIT compilation.

The add_buf function is a convenience wrapper that does the work of
creating a memfd for the caller.

Signed-off-by: Alastair Robertson <ajor@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241211164030.573042-3-ajor@meta.com

tools/lib/bpf/libbpf.h

@@ -1796,9 +1796,14 @@ struct bpf_linker_file_opts {
 struct bpf_linker;
 
 LIBBPF_API struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts *opts);
+LIBBPF_API struct bpf_linker *bpf_linker__new_fd(int fd, struct bpf_linker_opts *opts);
 LIBBPF_API int bpf_linker__add_file(struct bpf_linker *linker,
 				    const char *filename,
 				    const struct bpf_linker_file_opts *opts);
+LIBBPF_API int bpf_linker__add_fd(struct bpf_linker *linker, int fd,
+				  const struct bpf_linker_file_opts *opts);
+LIBBPF_API int bpf_linker__add_buf(struct bpf_linker *linker, void *buf, size_t buf_sz,
+				   const struct bpf_linker_file_opts *opts);
 LIBBPF_API int bpf_linker__finalize(struct bpf_linker *linker);
 LIBBPF_API void bpf_linker__free(struct bpf_linker *linker);
 

tools/lib/bpf/libbpf.map

@@ -432,4 +432,8 @@ LIBBPF_1.5.0 {
 } LIBBPF_1.4.0;
 
 LIBBPF_1.6.0 {
+	global:
+		bpf_linker__add_buf;
+		bpf_linker__add_fd;
+		bpf_linker__new_fd;
 } LIBBPF_1.5.0;

tools/lib/bpf/linker.c

@@ -4,6 +4,10 @@
  *
  * Copyright (c) 2021 Facebook
  */
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdio.h>
@@ -16,6 +20,7 @@
 #include <elf.h>
 #include <libelf.h>
 #include <fcntl.h>
+#include <sys/mman.h>
 #include "libbpf.h"
 #include "btf.h"
 #include "libbpf_internal.h"
@@ -152,6 +157,8 @@ struct bpf_linker {
 	/* global (including extern) ELF symbols */
 	int glob_sym_cnt;
 	struct glob_sym *glob_syms;
+
+	bool fd_is_owned;
 };
 
 #define pr_warn_elf(fmt, ...)									\
@@ -159,6 +166,9 @@ struct bpf_linker {
 
 static int init_output_elf(struct bpf_linker *linker);
 
+static int bpf_linker_add_file(struct bpf_linker *linker, int fd,
+			       const char *filename);
+
 static int linker_load_obj_file(struct bpf_linker *linker,
 				struct src_obj *obj);
 static int linker_sanity_check_elf(struct src_obj *obj);
@@ -190,7 +200,7 @@ void bpf_linker__free(struct bpf_linker *linker)
 	if (linker->elf)
 		elf_end(linker->elf);
 
-	if (linker->fd >= 0)
+	if (linker->fd >= 0 && linker->fd_is_owned)
 		close(linker->fd);
 
 	strset__free(linker->strtab_strs);
@@ -244,6 +254,49 @@ struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts
 		pr_warn("failed to create '%s': %d\n", filename, err);
 		goto err_out;
 	}
+	linker->fd_is_owned = true;
+
+	err = init_output_elf(linker);
+	if (err)
+		goto err_out;
+
+	return linker;
+
+err_out:
+	bpf_linker__free(linker);
+	return errno = -err, NULL;
+}
+
+struct bpf_linker *bpf_linker__new_fd(int fd, struct bpf_linker_opts *opts)
+{
+	struct bpf_linker *linker;
+	char filename[32];
+	int err;
+
+	if (fd < 0)
+		return errno = EINVAL, NULL;
+
+	if (!OPTS_VALID(opts, bpf_linker_opts))
+		return errno = EINVAL, NULL;
+
+	if (elf_version(EV_CURRENT) == EV_NONE) {
+		pr_warn_elf("libelf initialization failed");
+		return errno = EINVAL, NULL;
+	}
+
+	linker = calloc(1, sizeof(*linker));
+	if (!linker)
+		return errno = ENOMEM, NULL;
+
+	snprintf(filename, sizeof(filename), "fd:%d", fd);
+	linker->filename = strdup(filename);
+	if (!linker->filename) {
+		err = -ENOMEM;
+		goto err_out;
+	}
+
+	linker->fd = fd;
+	linker->fd_is_owned = false;
 
 	err = init_output_elf(linker);
 	if (err)
@@ -435,24 +488,11 @@ static int init_output_elf(struct bpf_linker *linker)
 	return 0;
 }
 
-int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
+static int bpf_linker_add_file(struct bpf_linker *linker, int fd,
-			 const struct bpf_linker_file_opts *opts)
+			       const char *filename)
 {
 	struct src_obj obj = {};
-	int err = 0, fd;
+	int err = 0;
-
-	if (!OPTS_VALID(opts, bpf_linker_file_opts))
-		return libbpf_err(-EINVAL);
-
-	if (!linker->elf)
-		return libbpf_err(-EINVAL);
-
-	fd = open(filename, O_RDONLY | O_CLOEXEC);
-	if (fd < 0) {
-		err = -errno;
-		pr_warn("failed to open file '%s': %s\n", filename, errstr(err));
-		return libbpf_err(err);
-	}
 
 	obj.filename = filename;
 	obj.fd = fd;
@@ -472,12 +512,91 @@ int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
 	free(obj.sym_map);
 	if (obj.elf)
 		elf_end(obj.elf);
-	if (obj.fd >= 0)
-		close(obj.fd);
 
+	return err;
+}
+
+int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
+			 const struct bpf_linker_file_opts *opts)
+{
+	int fd, err;
+
+	if (!OPTS_VALID(opts, bpf_linker_file_opts))
+		return libbpf_err(-EINVAL);
+
+	if (!linker->elf)
+		return libbpf_err(-EINVAL);
+
+	fd = open(filename, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		err = -errno;
+		pr_warn("failed to open file '%s': %s\n", filename, errstr(err));
 		return libbpf_err(err);
 	}
 
+	err = bpf_linker_add_file(linker, fd, filename);
+	close(fd);
+	return libbpf_err(err);
+}
+
+int bpf_linker__add_fd(struct bpf_linker *linker, int fd,
+		       const struct bpf_linker_file_opts *opts)
+{
+	char filename[32];
+	int err;
+
+	if (!OPTS_VALID(opts, bpf_linker_file_opts))
+		return libbpf_err(-EINVAL);
+
+	if (!linker->elf)
+		return libbpf_err(-EINVAL);
+
+	if (fd < 0)
+		return libbpf_err(-EINVAL);
+
+	snprintf(filename, sizeof(filename), "fd:%d", fd);
+	err = bpf_linker_add_file(linker, fd, filename);
+	return libbpf_err(err);
+}
+
+int bpf_linker__add_buf(struct bpf_linker *linker, void *buf, size_t buf_sz,
+			const struct bpf_linker_file_opts *opts)
+{
+	char filename[32];
+	int fd, written, ret;
+
+	if (!OPTS_VALID(opts, bpf_linker_file_opts))
+		return libbpf_err(-EINVAL);
+
+	if (!linker->elf)
+		return libbpf_err(-EINVAL);
+
+	snprintf(filename, sizeof(filename), "mem:%p+%zu", buf, buf_sz);
+
+	fd = memfd_create(filename, 0);
+	if (fd < 0) {
+		ret = -errno;
+		pr_warn("failed to create memfd '%s': %s\n", filename, errstr(ret));
+		return libbpf_err(ret);
+	}
+
+	written = 0;
+	while (written < buf_sz) {
+		ret = write(fd, buf, buf_sz);
+		if (ret < 0) {
+			ret = -errno;
+			pr_warn("failed to write '%s': %s\n", filename, errstr(ret));
+			goto err_out;
+		}
+		written += ret;
+	}
+
+	ret = bpf_linker_add_file(linker, fd, filename);
+err_out:
+	close(fd);
+	return libbpf_err(ret);
+}
+
 static bool is_dwarf_sec_name(const char *name)
 {
 	/* approximation, but the actual list is too long */
@@ -2687,9 +2806,10 @@ int bpf_linker__finalize(struct bpf_linker *linker)
 	}
 
 	elf_end(linker->elf);
-	close(linker->fd);
-
 	linker->elf = NULL;
+
+	if (linker->fd_is_owned)
+		close(linker->fd);
 	linker->fd = -1;
 
 	return 0;