Merge branch 'libbpf-extend-linker-api-to-support-in-memory-elf-files'

Alastair Robertson says:

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

This gives API consumers the option of using anonymous files/memfds to
avoid writing temporary ELFs to disk, which will be useful for performing
linking as part of bpftrace's JIT compilation.

v3:
- Removed "filename" option. Now always generate our own filename for
  passed-in FDs and buffers.
- Use a common function (bpf_linker_add_file) for shared
  implementation of bpf_linker__add_file, bpf_linker__add_fd and
  bpf_linker__add_buf.
====================

Link: https://patch.msgid.link/20241211164030.573042-1-ajor@meta.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

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"
@@ -152,15 +157,19 @@ 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, ...)									\
 	libbpf_print(LIBBPF_WARN, "libbpf: " fmt ": %s\n", ##__VA_ARGS__, elf_errmsg(-1))
 
-static int init_output_elf(struct bpf_linker *linker, const char *file);
+static int init_output_elf(struct bpf_linker *linker);
 
-static int linker_load_obj_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);
+
+static int linker_load_obj_file(struct bpf_linker *linker,
 				struct src_obj *obj);
 static int linker_sanity_check_elf(struct src_obj *obj);
 static int linker_sanity_check_elf_symtab(struct src_obj *obj, struct src_sec *sec);
@@ -191,7 +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);
@@ -233,9 +242,63 @@ struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts
 	if (!linker)
 		return errno = ENOMEM, NULL;
 
-	linker->fd = -1;
+	linker->filename = strdup(filename);
+	if (!linker->filename) {
+		err = -ENOMEM;
+		goto err_out;
+	}
 
-	err = init_output_elf(linker, filename);
+	linker->fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0644);
+	if (linker->fd < 0) {
+		err = -errno;
+		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)
 		goto err_out;
 
@@ -294,23 +357,12 @@ static Elf64_Sym *add_new_sym(struct bpf_linker *linker, size_t *sym_idx)
 	return sym;
 }
 
-static int init_output_elf(struct bpf_linker *linker, const char *file)
+static int init_output_elf(struct bpf_linker *linker)
 {
 	int err, str_off;
 	Elf64_Sym *init_sym;
 	struct dst_sec *sec;
 
-	linker->filename = strdup(file);
-	if (!linker->filename)
-		return -ENOMEM;
-
-	linker->fd = open(file, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, 0644);
-	if (linker->fd < 0) {
-		err = -errno;
-		pr_warn("failed to create '%s': %s\n", file, errstr(err));
-		return err;
-	}
-
 	linker->elf = elf_begin(linker->fd, ELF_C_WRITE, NULL);
 	if (!linker->elf) {
 		pr_warn_elf("failed to create ELF object");
@@ -436,19 +488,16 @@ static int init_output_elf(struct bpf_linker *linker, const char *file)
 	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;
 
-	if (!OPTS_VALID(opts, bpf_linker_file_opts))
-		return libbpf_err(-EINVAL);
+	obj.filename = filename;
+	obj.fd = fd;
 
-	if (!linker->elf)
-		return libbpf_err(-EINVAL);
-
-	err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
+	err = err ?: linker_load_obj_file(linker, &obj);
 	err = err ?: linker_append_sec_data(linker, &obj);
 	err = err ?: linker_append_elf_syms(linker, &obj);
 	err = err ?: linker_append_elf_relos(linker, &obj);
@@ -463,12 +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 */
@@ -534,8 +662,7 @@ static struct src_sec *add_src_sec(struct src_obj *obj, const char *sec_name)
 	return sec;
 }
 
-static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
-				const struct bpf_linker_file_opts *opts,
+static int linker_load_obj_file(struct bpf_linker *linker,
 				struct src_obj *obj)
 {
 	int err = 0;
@@ -554,26 +681,18 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 #error "Unknown __BYTE_ORDER__"
 #endif
 
-	pr_debug("linker: adding object file '%s'...\n", filename);
+	pr_debug("linker: adding object file '%s'...\n", obj->filename);
 
-	obj->filename = filename;
-
-	obj->fd = open(filename, O_RDONLY | O_CLOEXEC);
-	if (obj->fd < 0) {
-		err = -errno;
-		pr_warn("failed to open file '%s': %s\n", filename, errstr(err));
-		return err;
-	}
 	obj->elf = elf_begin(obj->fd, ELF_C_READ_MMAP, NULL);
 	if (!obj->elf) {
-		pr_warn_elf("failed to parse ELF file '%s'", filename);
+		pr_warn_elf("failed to parse ELF file '%s'", obj->filename);
 		return -EINVAL;
 	}
 
 	/* Sanity check ELF file high-level properties */
 	ehdr = elf64_getehdr(obj->elf);
 	if (!ehdr) {
-		pr_warn_elf("failed to get ELF header for %s", filename);
+		pr_warn_elf("failed to get ELF header for %s", obj->filename);
 		return -EINVAL;
 	}
 
@@ -581,7 +700,7 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 	obj_byteorder = ehdr->e_ident[EI_DATA];
 	if (obj_byteorder != ELFDATA2LSB && obj_byteorder != ELFDATA2MSB) {
 		err = -EOPNOTSUPP;
-		pr_warn("unknown byte order of ELF file %s\n", filename);
+		pr_warn("unknown byte order of ELF file %s\n", obj->filename);
 		return err;
 	}
 	if (link_byteorder == ELFDATANONE) {
@@ -591,7 +710,7 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 			 obj_byteorder == ELFDATA2MSB ? "big" : "little");
 	} else if (link_byteorder != obj_byteorder) {
 		err = -EOPNOTSUPP;
-		pr_warn("byte order mismatch with ELF file %s\n", filename);
+		pr_warn("byte order mismatch with ELF file %s\n", obj->filename);
 		return err;
 	}
 
@@ -599,12 +718,12 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 	    || ehdr->e_machine != EM_BPF
 	    || ehdr->e_ident[EI_CLASS] != ELFCLASS64) {
 		err = -EOPNOTSUPP;
-		pr_warn_elf("unsupported kind of ELF file %s", filename);
+		pr_warn_elf("unsupported kind of ELF file %s", obj->filename);
 		return err;
 	}
 
 	if (elf_getshdrstrndx(obj->elf, &obj->shstrs_sec_idx)) {
-		pr_warn_elf("failed to get SHSTRTAB section index for %s", filename);
+		pr_warn_elf("failed to get SHSTRTAB section index for %s", obj->filename);
 		return -EINVAL;
 	}
 
@@ -616,21 +735,21 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 		shdr = elf64_getshdr(scn);
 		if (!shdr) {
 			pr_warn_elf("failed to get section #%zu header for %s",
-				    sec_idx, filename);
+				    sec_idx, obj->filename);
 			return -EINVAL;
 		}
 
 		sec_name = elf_strptr(obj->elf, obj->shstrs_sec_idx, shdr->sh_name);
 		if (!sec_name) {
 			pr_warn_elf("failed to get section #%zu name for %s",
-				    sec_idx, filename);
+				    sec_idx, obj->filename);
 			return -EINVAL;
 		}
 
 		data = elf_getdata(scn, 0);
 		if (!data) {
 			pr_warn_elf("failed to get section #%zu (%s) data from %s",
-				    sec_idx, sec_name, filename);
+				    sec_idx, sec_name, obj->filename);
 			return -EINVAL;
 		}
 
@@ -666,7 +785,7 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 				err = libbpf_get_error(obj->btf);
 				if (err) {
 					pr_warn("failed to parse .BTF from %s: %s\n",
-						filename, errstr(err));
+						obj->filename, errstr(err));
 					return err;
 				}
 				sec->skipped = true;
@@ -677,7 +796,7 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 				err = libbpf_get_error(obj->btf_ext);
 				if (err) {
 					pr_warn("failed to parse .BTF.ext from '%s': %s\n",
-						filename, errstr(err));
+						obj->filename, errstr(err));
 					return err;
 				}
 				sec->skipped = true;
@@ -694,7 +813,7 @@ static int linker_load_obj_file(struct bpf_linker *linker, const char *filename,
 			break;
 		default:
 			pr_warn("unrecognized section #%zu (%s) in %s\n",
-				sec_idx, sec_name, filename);
+				sec_idx, sec_name, obj->filename);
 			err = -EINVAL;
 			return err;
 		}
@@ -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;