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: a6cc6b3 ("bpftool: Provide a helper method for accessing skeleton's embedded ELF data")
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20231007044439.25171-1-irogers@google.com

Author: captain5050

tools/bpf/bpftool/gen.c

@@ -1209,7 +1209,7 @@ static int do_skeleton(int argc, char **argv)
 	codegen("\
 		\n\
 									    \n\
-			s->data = %2$s__elf_bytes(&s->data_sz);		    \n\
+			s->data = %1$s__elf_bytes(&s->data_sz);		    \n\
 									    \n\
 			obj->skeleton = s;				    \n\
 			return 0;					    \n\
@@ -1218,19 +1218,22 @@ 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);
 
 	codegen("\
 		\n\
 		\";							    \n\
+									    \n\
+			*sz = sizeof(data) - 1;				    \n\
+			return (const void *)data;			    \n\
 		}							    \n\
 									    \n\
 		#ifdef __cplusplus					    \n\