linux-kernel - [PATCH v4 1/2] bpftool: Align output skeleton ELF code

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Message-Id: <20231004222323.3503030-1-irogers@google.com>
Date:   Wed,  4 Oct 2023 15:23:22 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Ian Rogers <irogers@...gle.com>,
        Alan Maguire <alan.maguire@...cle.com>
Subject: [PATCH v4 1/2] bpftool: Align output skeleton ELF code

libbpf accesses the ELF data requiring at least 8 byte alignment,
however, the data is generated into a C string that doesn't guarantee
alignment. Fix this by assigning to an aligned char array. Use sizeof
on the array, less one for the \0 terminator, rather than generating a
constant.

Fixes: a6cc6b34b93e ("bpftool: Provide a helper method for accessing skeleton's embedded ELF data")
Signed-off-by: Ian Rogers <irogers@...gle.com>
Reviewed-by: Alan Maguire <alan.maguire@...cle.com>
---
 tools/bpf/bpftool/gen.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 2883660d6b67..b8ebcee9bc56 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -1209,7 +1209,7 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 									    \n\
-			s->data = (void *)%2$s__elf_bytes(&s->data_sz);	    \n\
+			s->data = (void *)%1$s__elf_bytes(&s->data_sz);	    \n\
 									    \n\
 			obj->skeleton = s;				    \n\
 			return 0;					    \n\
@@ -1218,12 +1218,12 @@ static int do_skeleton(int argc, char **argv)
 			return err;					    \n\
 		}							    \n\
 									    \n\
-		static inline const void *%2$s__elf_bytes(size_t *sz)	    \n\
+		static inline const void *%1$s__elf_bytes(size_t *sz)	    \n\
 		{							    \n\
-			*sz = %1$d;					    \n\
-			return (const void *)\"\\			    \n\
-		"
-		, file_sz, obj_name);
+			static const char data[] __attribute__((__aligned__(8))) = \"\\\n\
+		",
+		obj_name
+	);
 
 	/* embed contents of BPF object file */
 	print_hex(obj_data, file_sz);
@@ -1231,6 +1231,9 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 		\";							    \n\
+									    \n\
+			*sz = sizeof(data) - 1;				    \n\
+			return (const void *)data;			    \n\
 		}							    \n\
 									    \n\
 		#ifdef __cplusplus					    \n\
-- 
2.42.0.609.gbb76f46606-goog