Commit 6d5e5e5d authored by Alastair Robertson's avatar Alastair Robertson Committed by Andrii Nakryiko
Browse files

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: default avatarAlastair Robertson <ajor@meta.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241211164030.573042-3-ajor@meta.com
parent b6417129

tools/lib/bpf/libbpf.h

+5 −0
Original line number Diff line number Diff line
@@ -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

+4 −0
Original line number Diff line number Diff line
@@ -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

+141 −21
Original line number Diff line number Diff line
@@ -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,

			 const struct bpf_linker_file_opts *opts)

static int bpf_linker_add_file(struct bpf_linker *linker, int fd,

			       const char *filename)

{

	struct src_obj obj = {};

	int err = 0, fd;



	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);

	}

	int err = 0;



	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;