[PATCH v2 bpf-next 0/3] Fix libbpf BPF skeleton forward/backward compat

[PATCH v2 bpf-next 0/3] Fix libbpf BPF skeleton forward/backward compat

[Andrii Nakryiko]

Fix recently identified (but long standing) bug with handling BPF skeleton
forward and backward compatibility. On libbpf side, even though BPF skeleton
was always designed to be forward and backwards compatible through recording
actual size of constrituents of BPF skeleton itself (map/prog/var skeleton
definitions), libbpf implementation did implicitly hard-code those sizes by
virtue of using a trivial array access syntax.

This issue will only affect libbpf used as a shared library. Statically
compiled libbpfs will always be in sync with BPF skeleton, bypassing this
problem altogether.

This patch set fixes libbpf, but also mitigates the problem for old libbpf
versions by teaching bpftool to generate more conservative BPF skeleton,
if possible (i.e., if there are no struct_ops maps defined).

v1->v2:
  - fix SOB, add acks, typo fixes (Quentin, Eduard);
  - improve reporting of skipped map auto-attachment (Alan, Eduard).

Andrii Nakryiko (3):
  bpftool: improve skeleton backwards compat with old buggy libbpfs
  libbpf: fix BPF skeleton forward/backward compat handling
  libbpf: improve old BPF skeleton handling for map auto-attach

 tools/bpf/bpftool/gen.c | 46 ++++++++++++++++++--------
 tools/lib/bpf/libbpf.c  | 71 +++++++++++++++++++++++------------------
 2 files changed, 72 insertions(+), 45 deletions(-)

-- 
2.43.0

[PATCH v2 bpf-next 1/3] bpftool: improve skeleton backwards compat with old buggy libbpfs

[Andrii Nakryiko]

Old versions of libbpf don't handle varying sizes of bpf_map_skeleton
struct correctly. As such, BPF skeleton generated by newest bpftool
might not be compatible with older libbpf (though only when libbpf is
used as a shared library), even though it, by design, should.

Going forward libbpf will be fixed, plus we'll release bug fixed
versions of relevant old libbpfs, but meanwhile try to mitigate from
bpftool side by conservatively assuming older and smaller definition of
bpf_map_skeleton, if possible. Meaning, if there are no struct_ops maps.

If there are struct_ops, then presumably user would like to have
auto-attaching logic and struct_ops map link placeholders, so use the
full bpf_map_skeleton definition in that case.

Acked-by: Quentin Monnet <qmo@kernel.org>
Co-developed-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/bpf/bpftool/gen.c | 46 ++++++++++++++++++++++++++++-------------
 1 file changed, 32 insertions(+), 14 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 51eaed76db97..70aaa32ddcc9 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -852,24 +852,41 @@ codegen_maps_skeleton(struct bpf_object *obj, size_t map_cnt, bool mmaped, bool
 {
 	struct bpf_map *map;
 	char ident[256];
-	size_t i;
+	size_t i, map_sz;
 
 	if (!map_cnt)
 		return;
 
+	/* for backward compatibility with old libbpf versions that don't
+	 * handle new BPF skeleton with new struct bpf_map_skeleton definition
+	 * that includes link field, avoid specifying new increased size,
+	 * unless we absolutely have to (i.e., if there are struct_ops maps
+	 * present)
+	 */
+	map_sz = offsetof(struct bpf_map_skeleton, link);
+	if (populate_links) {
+		bpf_object__for_each_map(map, obj) {
+			if (bpf_map__type(map) == BPF_MAP_TYPE_STRUCT_OPS) {
+				map_sz = sizeof(struct bpf_map_skeleton);
+				break;
+			}
+		}
+	}
+
 	codegen("\
 		\n\
-									\n\
+								    \n\
 			/* maps */				    \n\
 			s->map_cnt = %zu;			    \n\
-			s->map_skel_sz = sizeof(*s->maps);	    \n\
-			s->maps = (struct bpf_map_skeleton *)calloc(s->map_cnt, s->map_skel_sz);\n\
+			s->map_skel_sz = %zu;			    \n\
+			s->maps = (struct bpf_map_skeleton *)calloc(s->map_cnt,\n\
+					sizeof(*s->maps) > %zu ? sizeof(*s->maps) : %zu);\n\
 			if (!s->maps) {				    \n\
 				err = -ENOMEM;			    \n\
 				goto err;			    \n\
 			}					    \n\
 		",
-		map_cnt
+		map_cnt, map_sz, map_sz, map_sz
 	);
 	i = 0;
 	bpf_object__for_each_map(map, obj) {
@@ -878,23 +895,22 @@ codegen_maps_skeleton(struct bpf_object *obj, size_t map_cnt, bool mmaped, bool
 
 		codegen("\
 			\n\
-									\n\
-				s->maps[%zu].name = \"%s\";	    \n\
-				s->maps[%zu].map = &obj->maps.%s;   \n\
+								    \n\
+				map = (struct bpf_map_skeleton *)((char *)s->maps + %zu * s->map_skel_sz);\n\
+				map->name = \"%s\";		    \n\
+				map->map = &obj->maps.%s;	    \n\
 			",
-			i, bpf_map__name(map), i, ident);
+			i, bpf_map__name(map), ident);
 		/* memory-mapped internal maps */
 		if (mmaped && is_mmapable_map(map, ident, sizeof(ident))) {
-			printf("\ts->maps[%zu].mmaped = (void **)&obj->%s;\n",
-				i, ident);
+			printf("\tmap->mmaped = (void **)&obj->%s;  \n", ident);
 		}
 
 		if (populate_links && bpf_map__type(map) == BPF_MAP_TYPE_STRUCT_OPS) {
 			codegen("\
 				\n\
-					s->maps[%zu].link = &obj->links.%s;\n\
-				",
-				i, ident);
+					map->link = &obj->links.%s; \n\
+				", ident);
 		}
 		i++;
 	}
@@ -1463,6 +1479,7 @@ static int do_skeleton(int argc, char **argv)
 		%1$s__create_skeleton(struct %1$s *obj)			    \n\
 		{							    \n\
 			struct bpf_object_skeleton *s;			    \n\
+			struct bpf_map_skeleton *map __attribute__((unused));\n\
 			int err;					    \n\
 									    \n\
 			s = (struct bpf_object_skeleton *)calloc(1, sizeof(*s));\n\
@@ -1753,6 +1770,7 @@ static int do_subskeleton(int argc, char **argv)
 		{							    \n\
 			struct %1$s *obj;				    \n\
 			struct bpf_object_subskeleton *s;		    \n\
+			struct bpf_map_skeleton *map __attribute__((unused));\n\
 			int err;					    \n\
 									    \n\
 			obj = (struct %1$s *)calloc(1, sizeof(*obj));	    \n\
-- 
2.43.0

Re: [PATCH v2 bpf-next 1/3] bpftool: improve skeleton backwards compat with old buggy libbpfs

[Eduard Zingerman]

On Mon, 2024-07-08 at 13:45 -0700, Andrii Nakryiko wrote:
> Old versions of libbpf don't handle varying sizes of bpf_map_skeleton
> struct correctly. As such, BPF skeleton generated by newest bpftool
> might not be compatible with older libbpf (though only when libbpf is
> used as a shared library), even though it, by design, should.
> 
> Going forward libbpf will be fixed, plus we'll release bug fixed
> versions of relevant old libbpfs, but meanwhile try to mitigate from
> bpftool side by conservatively assuming older and smaller definition of
> bpf_map_skeleton, if possible. Meaning, if there are no struct_ops maps.
> 
> If there are struct_ops, then presumably user would like to have
> auto-attaching logic and struct_ops map link placeholders, so use the
> full bpf_map_skeleton definition in that case.
> 
> Acked-by: Quentin Monnet <qmo@kernel.org>
> Co-developed-by: Mykyta Yatsenko <yatsenko@meta.com>
> Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

Re: [PATCH v2 bpf-next 1/3] bpftool: improve skeleton backwards compat with old buggy libbpfs

[Eduard Zingerman]

On Mon, 2024-07-08 at 13:45 -0700, Andrii Nakryiko wrote:

[...]

> @@ -878,23 +895,22 @@ codegen_maps_skeleton(struct bpf_object *obj, size_t map_cnt, bool mmaped, bool
>  
>  		codegen("\
>  			\n\
> -									\n\
> -				s->maps[%zu].name = \"%s\";	    \n\
> -				s->maps[%zu].map = &obj->maps.%s;   \n\
> +								    \n\
> +				map = (struct bpf_map_skeleton *)((char *)s->maps + %zu * s->map_skel_sz);\n\
> +				map->name = \"%s\";		    \n\
> +				map->map = &obj->maps.%s;	    \n\
>  			",
> -			i, bpf_map__name(map), i, ident);
> +			i, bpf_map__name(map), ident);
>  		/* memory-mapped internal maps */
>  		if (mmaped && is_mmapable_map(map, ident, sizeof(ident))) {
> -			printf("\ts->maps[%zu].mmaped = (void **)&obj->%s;\n",
> -				i, ident);
> +			printf("\tmap->mmaped = (void **)&obj->%s;  \n", ident);
                                                                 ^^^^
                                             nit: this still prints extra white space

[...]

Re: [PATCH v2 bpf-next 1/3] bpftool: improve skeleton backwards compat with old buggy libbpfs

[Andrii Nakryiko]

On Mon, Jul 8, 2024 at 3:54 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> On Mon, 2024-07-08 at 13:45 -0700, Andrii Nakryiko wrote:
>
> [...]
>
> > @@ -878,23 +895,22 @@ codegen_maps_skeleton(struct bpf_object *obj, size_t map_cnt, bool mmaped, bool
> >
> >               codegen("\
> >                       \n\
> > -                                                                     \n\
> > -                             s->maps[%zu].name = \"%s\";         \n\
> > -                             s->maps[%zu].map = &obj->maps.%s;   \n\
> > +                                                                 \n\
> > +                             map = (struct bpf_map_skeleton *)((char *)s->maps + %zu * s->map_skel_sz);\n\
> > +                             map->name = \"%s\";                 \n\
> > +                             map->map = &obj->maps.%s;           \n\
> >                       ",
> > -                     i, bpf_map__name(map), i, ident);
> > +                     i, bpf_map__name(map), ident);
> >               /* memory-mapped internal maps */
> >               if (mmaped && is_mmapable_map(map, ident, sizeof(ident))) {
> > -                     printf("\ts->maps[%zu].mmaped = (void **)&obj->%s;\n",
> > -                             i, ident);
> > +                     printf("\tmap->mmaped = (void **)&obj->%s;  \n", ident);
>                                                                  ^^^^
>                                              nit: this still prints extra white space

ah, that's just printf(), not codegen(), missed that. I don't want to
send another revision for this. Hopefully whoever applies can just
remove those two spaces, if not it's no big deal either way and we can
fix it up later.

>
> [...]

[PATCH v2 bpf-next 2/3] libbpf: fix BPF skeleton forward/backward compat handling

[Andrii Nakryiko]

BPF skeleton was designed from day one to be extensible. Generated BPF
skeleton code specifies actual sizes of map/prog/variable skeletons for
that reason and libbpf is supposed to work with newer/older versions
correctly.

Unfortunately, it was missed that we implicitly embed hard-coded most
up-to-date (according to libbpf's version of libbpf.h header used to
compile BPF skeleton header) sizes of those structs, which can differ
from the actual sizes at runtime when libbpf is used as a shared
library.

We have a few places were we just index array of maps/progs/vars, which
implicitly uses these potentially invalid sizes of structs.

This patch aims to fix this problem going forward. Once this lands,
we'll backport these changes in Github repo to create patched releases
for older libbpfs.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Fixes: d66562fba1ce ("libbpf: Add BPF object skeleton support")
Fixes: 430025e5dca5 ("libbpf: Add subskeleton scaffolding")
Fixes: 08ac454e258 ("libbpf: Auto-attach struct_ops BPF maps in BPF skeleton")
Co-developed-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c | 47 ++++++++++++++++++++++++------------------
 1 file changed, 27 insertions(+), 20 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 30f121754d83..8625c948f60a 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -13712,14 +13712,15 @@ int libbpf_num_possible_cpus(void)
 
 static int populate_skeleton_maps(const struct bpf_object *obj,
 				  struct bpf_map_skeleton *maps,
-				  size_t map_cnt)
+				  size_t map_cnt, size_t map_skel_sz)
 {
 	int i;
 
 	for (i = 0; i < map_cnt; i++) {
-		struct bpf_map **map = maps[i].map;
-		const char *name = maps[i].name;
-		void **mmaped = maps[i].mmaped;
+		struct bpf_map_skeleton *map_skel = (void *)maps + i * map_skel_sz;
+		struct bpf_map **map = map_skel->map;
+		const char *name = map_skel->name;
+		void **mmaped = map_skel->mmaped;
 
 		*map = bpf_object__find_map_by_name(obj, name);
 		if (!*map) {
@@ -13736,13 +13737,14 @@ static int populate_skeleton_maps(const struct bpf_object *obj,
 
 static int populate_skeleton_progs(const struct bpf_object *obj,
 				   struct bpf_prog_skeleton *progs,
-				   size_t prog_cnt)
+				   size_t prog_cnt, size_t prog_skel_sz)
 {
 	int i;
 
 	for (i = 0; i < prog_cnt; i++) {
-		struct bpf_program **prog = progs[i].prog;
-		const char *name = progs[i].name;
+		struct bpf_prog_skeleton *prog_skel = (void *)progs + i * prog_skel_sz;
+		struct bpf_program **prog = prog_skel->prog;
+		const char *name = prog_skel->name;
 
 		*prog = bpf_object__find_program_by_name(obj, name);
 		if (!*prog) {
@@ -13783,13 +13785,13 @@ int bpf_object__open_skeleton(struct bpf_object_skeleton *s,
 	}
 
 	*s->obj = obj;
-	err = populate_skeleton_maps(obj, s->maps, s->map_cnt);
+	err = populate_skeleton_maps(obj, s->maps, s->map_cnt, s->map_skel_sz);
 	if (err) {
 		pr_warn("failed to populate skeleton maps for '%s': %d\n", s->name, err);
 		return libbpf_err(err);
 	}
 
-	err = populate_skeleton_progs(obj, s->progs, s->prog_cnt);
+	err = populate_skeleton_progs(obj, s->progs, s->prog_cnt, s->prog_skel_sz);
 	if (err) {
 		pr_warn("failed to populate skeleton progs for '%s': %d\n", s->name, err);
 		return libbpf_err(err);
@@ -13819,20 +13821,20 @@ int bpf_object__open_subskeleton(struct bpf_object_subskeleton *s)
 		return libbpf_err(-errno);
 	}
 
-	err = populate_skeleton_maps(s->obj, s->maps, s->map_cnt);
+	err = populate_skeleton_maps(s->obj, s->maps, s->map_cnt, s->map_skel_sz);
 	if (err) {
 		pr_warn("failed to populate subskeleton maps: %d\n", err);
 		return libbpf_err(err);
 	}
 
-	err = populate_skeleton_progs(s->obj, s->progs, s->prog_cnt);
+	err = populate_skeleton_progs(s->obj, s->progs, s->prog_cnt, s->prog_skel_sz);
 	if (err) {
 		pr_warn("failed to populate subskeleton maps: %d\n", err);
 		return libbpf_err(err);
 	}
 
 	for (var_idx = 0; var_idx < s->var_cnt; var_idx++) {
-		var_skel = &s->vars[var_idx];
+		var_skel = (void *)s->vars + var_idx * s->var_skel_sz;
 		map = *var_skel->map;
 		map_type_id = bpf_map__btf_value_type_id(map);
 		map_type = btf__type_by_id(btf, map_type_id);
@@ -13879,10 +13881,11 @@ int bpf_object__load_skeleton(struct bpf_object_skeleton *s)
 	}
 
 	for (i = 0; i < s->map_cnt; i++) {
-		struct bpf_map *map = *s->maps[i].map;
+		struct bpf_map_skeleton *map_skel = (void *)s->maps + i * s->map_skel_sz;
+		struct bpf_map *map = *map_skel->map;
 		size_t mmap_sz = bpf_map_mmap_sz(map);
 		int prot, map_fd = map->fd;
-		void **mmaped = s->maps[i].mmaped;
+		void **mmaped = map_skel->mmaped;
 
 		if (!mmaped)
 			continue;
@@ -13930,8 +13933,9 @@ int bpf_object__attach_skeleton(struct bpf_object_skeleton *s)
 	int i, err;
 
 	for (i = 0; i < s->prog_cnt; i++) {
-		struct bpf_program *prog = *s->progs[i].prog;
-		struct bpf_link **link = s->progs[i].link;
+		struct bpf_prog_skeleton *prog_skel = (void *)s->progs + i * s->prog_skel_sz;
+		struct bpf_program *prog = *prog_skel->prog;
+		struct bpf_link **link = prog_skel->link;
 
 		if (!prog->autoload || !prog->autoattach)
 			continue;
@@ -13970,8 +13974,9 @@ int bpf_object__attach_skeleton(struct bpf_object_skeleton *s)
 		return 0;
 
 	for (i = 0; i < s->map_cnt; i++) {
-		struct bpf_map *map = *s->maps[i].map;
-		struct bpf_link **link = s->maps[i].link;
+		struct bpf_map_skeleton *map_skel = (void *)s->maps + i * s->map_skel_sz;
+		struct bpf_map *map = *map_skel->map;
+		struct bpf_link **link = map_skel->link;
 
 		if (!map->autocreate || !map->autoattach)
 			continue;
@@ -14000,7 +14005,8 @@ void bpf_object__detach_skeleton(struct bpf_object_skeleton *s)
 	int i;
 
 	for (i = 0; i < s->prog_cnt; i++) {
-		struct bpf_link **link = s->progs[i].link;
+		struct bpf_prog_skeleton *prog_skel = (void *)s->progs + i * s->prog_skel_sz;
+		struct bpf_link **link = prog_skel->link;
 
 		bpf_link__destroy(*link);
 		*link = NULL;
@@ -14010,7 +14016,8 @@ void bpf_object__detach_skeleton(struct bpf_object_skeleton *s)
 		return;
 
 	for (i = 0; i < s->map_cnt; i++) {
-		struct bpf_link **link = s->maps[i].link;
+		struct bpf_map_skeleton *map_skel = (void *)s->maps + i * s->map_skel_sz;
+		struct bpf_link **link = map_skel->link;
 
 		if (link) {
 			bpf_link__destroy(*link);
-- 
2.43.0

[PATCH v2 bpf-next 3/3] libbpf: improve old BPF skeleton handling for map auto-attach

[Andrii Nakryiko]

Improve how we handle old BPF skeletons when it comes to BPF map
auto-attachment. Emit one warn-level message per each struct_ops map
that could have been auto-attached, if user provided recent enough BPF
skeleton version. Don't spam log if there are no relevant struct_ops
maps, though.

This should help users realize that they probably need to regenerate BPF
skeleton header with more recent bpftool/libbpf-cargo (or whatever other
means of BPF skeleton generation).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 8625c948f60a..a3be6f8fac09 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -13967,32 +13967,34 @@ int bpf_object__attach_skeleton(struct bpf_object_skeleton *s)
 		 */
 	}
 
-	/* Skeleton is created with earlier version of bpftool
-	 * which does not support auto-attachment
-	 */
-	if (s->map_skel_sz < sizeof(struct bpf_map_skeleton))
-		return 0;
 
 	for (i = 0; i < s->map_cnt; i++) {
 		struct bpf_map_skeleton *map_skel = (void *)s->maps + i * s->map_skel_sz;
 		struct bpf_map *map = *map_skel->map;
-		struct bpf_link **link = map_skel->link;
+		struct bpf_link **link;
 
 		if (!map->autocreate || !map->autoattach)
 			continue;
 
-		if (*link)
-			continue;
-
 		/* only struct_ops maps can be attached */
 		if (!bpf_map__is_struct_ops(map))
 			continue;
-		*link = bpf_map__attach_struct_ops(map);
 
+		/* skeleton is created with earlier version of bpftool, notify user */
+		if (s->map_skel_sz < offsetofend(struct bpf_map_skeleton, link)) {
+			pr_warn("map '%s': BPF skeleton version is old, skipping map auto-attachment...\n",
+				bpf_map__name(map));
+			continue;
+		}
+
+		link = map_skel->link;
+		if (*link)
+			continue;
+
+		*link = bpf_map__attach_struct_ops(map);
 		if (!*link) {
 			err = -errno;
-			pr_warn("map '%s': failed to auto-attach: %d\n",
-				bpf_map__name(map), err);
+			pr_warn("map '%s': failed to auto-attach: %d\n", bpf_map__name(map), err);
 			return libbpf_err(err);
 		}
 	}
-- 
2.43.0

Re: [PATCH v2 bpf-next 3/3] libbpf: improve old BPF skeleton handling for map auto-attach

[Eduard Zingerman]

On Mon, 2024-07-08 at 13:45 -0700, Andrii Nakryiko wrote:
> Improve how we handle old BPF skeletons when it comes to BPF map
> auto-attachment. Emit one warn-level message per each struct_ops map
> that could have been auto-attached, if user provided recent enough BPF
> skeleton version. Don't spam log if there are no relevant struct_ops
> maps, though.
> 
> This should help users realize that they probably need to regenerate BPF
> skeleton header with more recent bpftool/libbpf-cargo (or whatever other
> means of BPF skeleton generation).
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

Re: [PATCH v2 bpf-next 0/3] Fix libbpf BPF skeleton forward/backward compat

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Mon,  8 Jul 2024 13:45:37 -0700 you wrote:
> Fix recently identified (but long standing) bug with handling BPF skeleton
> forward and backward compatibility. On libbpf side, even though BPF skeleton
> was always designed to be forward and backwards compatible through recording
> actual size of constrituents of BPF skeleton itself (map/prog/var skeleton
> definitions), libbpf implementation did implicitly hard-code those sizes by
> virtue of using a trivial array access syntax.
> 
> [...]

Here is the summary with links:
  - [v2,bpf-next,1/3] bpftool: improve skeleton backwards compat with old buggy libbpfs
    https://git.kernel.org/bpf/bpf-next/c/06e71ad53488
  - [v2,bpf-next,2/3] libbpf: fix BPF skeleton forward/backward compat handling
    https://git.kernel.org/bpf/bpf-next/c/99fb9531886d
  - [v2,bpf-next,3/3] libbpf: improve old BPF skeleton handling for map auto-attach
    https://git.kernel.org/bpf/bpf-next/c/a459f4bb27f2

You are awesome, thank you!
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