[bpf-next,v3,2/2] libbpf: Extend linker API to support in-memory ELF files

Commit Message

Alastair Robertson Dec. 11, 2024, 4:40 p.m. UTC

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 <[e-mail]>
---
 tools/lib/bpf/libbpf.h   |   5 ++
 tools/lib/bpf/libbpf.map |   4 +
 tools/lib/bpf/linker.c   | 163 ++++++++++++++++++++++++++++++++++-----
 3 files changed, 152 insertions(+), 20 deletions(-)

Comments

Andrii Nakryiko Dec. 12, 2024, 11:29 p.m. UTC | #1

On Wed, Dec 11, 2024 at 8:40 AM Alastair Robertson <[e-mail]> wrote:
>
> 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 <[e-mail]>
> ---
>  tools/lib/bpf/libbpf.h   |   5 ++
>  tools/lib/bpf/libbpf.map |   4 +
>  tools/lib/bpf/linker.c   | 163 ++++++++++++++++++++++++++++++++++-----
>  3 files changed, 152 insertions(+), 20 deletions(-)
>

There were a bunch of errno mishandling issues and a small memory
leak. I fixed all that up and landed in bpf-next.

> +int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
> +                        const struct bpf_linker_file_opts *opts)
> +{
> +       int fd, ret;
> +
> +       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) {
> +               pr_warn("failed to open file '%s': %s\n", filename, errstr(errno));
> +               return -errno;

same issue with errno clobbering, fixed, but please be careful with
errno, pretty much anything can clobber errno in libc, except free()
and maybe a few more APIs

> +       }
> +
> +       ret = bpf_linker_add_file(linker, fd, filename);
> +
> +       close(fd);
> +
> +       return ret;

this needed to be libbpf_err(ret), because close() can clobber errno,
added while applying

> +}
> +
> +int bpf_linker__add_fd(struct bpf_linker *linker, int fd,
> +                      const struct bpf_linker_file_opts *opts)
> +{
> +       char filename[32];
> +       int ret;
> +
> +       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);
> +
> +       ret = bpf_linker_add_file(linker, fd, filename);
> +
> +       return ret;

here as well

> +}
> +
> +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) {

and here

> +               pr_warn("failed to create memfd '%s': %s\n", filename, errstr(errno));
> +               return -errno;
> +       }
> +
> +       written = 0;
> +       while (written < buf_sz) {
> +               ret = write(fd, buf, buf_sz);
> +               if (ret < 0) {

and here

and also you were leaking memfd here, I added jump to close(fd)

> +                       pr_warn("failed to write '%s': %s\n", filename, errstr(errno));
> +                       return -errno;
> +               }
> +               written += ret;
> +       }
> +
> +       ret = bpf_linker_add_file(linker, fd, filename);
> +
> +       close(fd);
> +
> +       return ret;

and here


> +}
> +
>  static bool is_dwarf_sec_name(const char *name)
>  {
>         /* approximation, but the actual list is too long */
> @@ -2686,9 +2808,10 @@ 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;
> --
> 2.43.5
>

Patch

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index b2ce3a72b11d..d45807103565 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/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);
 
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 54b6f312cfa8..a8b2936a1646 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/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;
diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index eb2ac7afce01..6b3b86d98f6c 100644
--- a/tools/lib/bpf/linker.c
+++ b/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)
@@ -435,23 +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) {
-		pr_warn("failed to open file '%s': %s\n", filename, errstr(errno));
-		return -errno;
-	}
+	int err = 0;
 
 	obj.filename = filename;
 	obj.fd = fd;
@@ -471,12 +512,93 @@  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 libbpf_err(err);
 }
 
+int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
+			 const struct bpf_linker_file_opts *opts)
+{
+	int fd, ret;
+
+	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) {
+		pr_warn("failed to open file '%s': %s\n", filename, errstr(errno));
+		return -errno;
+	}
+
+	ret = bpf_linker_add_file(linker, fd, filename);
+
+	close(fd);
+
+	return ret;
+}
+
+int bpf_linker__add_fd(struct bpf_linker *linker, int fd,
+		       const struct bpf_linker_file_opts *opts)
+{
+	char filename[32];
+	int ret;
+
+	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);
+
+	ret = bpf_linker_add_file(linker, fd, filename);
+
+	return ret;
+}
+
+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) {
+		pr_warn("failed to create memfd '%s': %s\n", filename, errstr(errno));
+		return -errno;
+	}
+
+	written = 0;
+	while (written < buf_sz) {
+		ret = write(fd, buf, buf_sz);
+		if (ret < 0) {
+			pr_warn("failed to write '%s': %s\n", filename, errstr(errno));
+			return -errno;
+		}
+		written += ret;
+	}
+
+	ret = bpf_linker_add_file(linker, fd, filename);
+
+	close(fd);
+
+	return ret;
+}
+
 static bool is_dwarf_sec_name(const char *name)
 {
 	/* approximation, but the actual list is too long */
@@ -2686,9 +2808,10 @@  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;