libbpf: Fix section counting logic

e_shnum does include section #0 and as such is exactly the number of ELF
sections that we need to allocate memory for to use section indices as
array indices. Fix the off-by-one error.

This is purely accounting fix, previously we were overallocating one
too many array items. But no correctness errors otherwise.

Fixes: 25bbbd7a ("libbpf: Remove assumptions about uniqueness of .rodata/.data/.bss maps")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20211103173213.1376990-5-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -3190,11 +3190,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 	Elf_Scn *scn;
 	Elf64_Shdr *sh;
 
-	/* ELF section indices are 1-based, so allocate +1 element to keep
-	 * indexing simple. Also include 0th invalid section into sec_cnt for
-	 * simpler and more traditional iteration logic.
+	/* ELF section indices are 0-based, but sec #0 is special "invalid"
+	 * section. e_shnum does include sec #0, so e_shnum is the necessary
+	 * size of an array to keep all the sections.
 	 */
-	obj->efile.sec_cnt = 1 + obj->efile.ehdr->e_shnum;
+	obj->efile.sec_cnt = obj->efile.ehdr->e_shnum;
 	obj->efile.secs = calloc(obj->efile.sec_cnt, sizeof(*obj->efile.secs));
 	if (!obj->efile.secs)
 		return -ENOMEM;