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

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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
This commit is contained in:
Alastair Robertson 2024-12-11 08:40:30 -08:00 • committed by Andrii Nakryiko
parent b641712925
commit 6d5e5e5d7c
3 changed files with 150 additions and 21 deletions
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5
tools/lib/bpf/libbpf.h
View file

@ -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);

4
tools/lib/bpf/libbpf.map
View file

@ -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;

162
tools/lib/bpf/linker.c
View file

@ -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;