libbpf: Add pr_warn() for EINVAL cases in linker_sanity_check_elf

Before the change on `i686-linux` `systemd` build failed as:

    $ bpftool gen object src/core/bpf/socket_bind/socket-bind.bpf.o src/core/bpf/socket_bind/socket-bind.bpf.unstripped.o
    Error: failed to link 'src/core/bpf/socket_bind/socket-bind.bpf.unstripped.o': Invalid argument (22)

After the change it fails as:

    $ bpftool gen object src/core/bpf/socket_bind/socket-bind.bpf.o src/core/bpf/socket_bind/socket-bind.bpf.unstripped.o
    libbpf: ELF section #9 has inconsistent alignment addr=8 != d=4 in src/core/bpf/socket_bind/socket-bind.bpf.unstripped.o
    Error: failed to link 'src/core/bpf/socket_bind/socket-bind.bpf.unstripped.o': Invalid argument (22)

Now it's slightly easier to figure out what is wrong with an ELF file.
Signed-off-by: Sergei Trofimovich <slyich@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20231208215100.435876-1-slyich@gmail.com

tools/lib/bpf/linker.c

@@ -719,13 +719,25 @@ static int linker_sanity_check_elf(struct src_obj *obj)
 			return -EINVAL;
 		}
 
-		if (sec->shdr->sh_addralign && !is_pow_of_2(sec->shdr->sh_addralign))
+		if (sec->shdr->sh_addralign && !is_pow_of_2(sec->shdr->sh_addralign)) {
+			pr_warn("ELF section #%zu alignment %llu is non pow-of-2 alignment in %s\n",
+				sec->sec_idx, (long long unsigned)sec->shdr->sh_addralign,
+				obj->filename);
 			return -EINVAL;
-		if (sec->shdr->sh_addralign != sec->data->d_align)
+		}
+		if (sec->shdr->sh_addralign != sec->data->d_align) {
+			pr_warn("ELF section #%zu has inconsistent alignment addr=%llu != d=%llu in %s\n",
+				sec->sec_idx, (long long unsigned)sec->shdr->sh_addralign,
+				(long long unsigned)sec->data->d_align, obj->filename);
 			return -EINVAL;
+		}
 
-		if (sec->shdr->sh_size != sec->data->d_size)
+		if (sec->shdr->sh_size != sec->data->d_size) {
+			pr_warn("ELF section #%zu has inconsistent section size sh=%llu != d=%llu in %s\n",
+				sec->sec_idx, (long long unsigned)sec->shdr->sh_size,
+				(long long unsigned)sec->data->d_size, obj->filename);
 			return -EINVAL;
+		}
 
 		switch (sec->shdr->sh_type) {
 		case SHT_SYMTAB:
@@ -737,8 +749,12 @@ static int linker_sanity_check_elf(struct src_obj *obj)
 			break;
 		case SHT_PROGBITS:
 			if (sec->shdr->sh_flags & SHF_EXECINSTR) {
-				if (sec->shdr->sh_size % sizeof(struct bpf_insn) != 0)
+				if (sec->shdr->sh_size % sizeof(struct bpf_insn) != 0) {
+					pr_warn("ELF section #%zu has unexpected size alignment %llu in %s\n",
+						sec->sec_idx, (long long unsigned)sec->shdr->sh_size,
+						obj->filename);
 					return -EINVAL;
+				}
 			}
 			break;
 		case SHT_NOBITS: