[PATCH bpf-next v2] bpftool: Fix gen object segfault

[Rong Tao]

From: Rong Tao <rongtao@xxxxxxxx>

If the input file and output file are the same, the input file is cleared
due to opening, resulting in a NULL pointer access by libbpf.

    $ bpftool gen object prog.o prog.o
    libbpf: failed to get ELF header for prog.o: invalid `Elf' handle
    Segmentation fault

    (gdb) bt
    #0  0x0000000000450285 in linker_append_elf_syms (linker=0x4feda0, obj=0x7fffffffe100) at linker.c:1296
    #1  bpf_linker__add_file (linker=0x4feda0, filename=<optimized out>, opts=<optimized out>) at linker.c:453
    #2  0x000000000040c235 in do_object ()
    #3  0x00000000004021d7 in main ()
    (gdb) frame 0
    #0  0x0000000000450285 in linker_append_elf_syms (linker=0x4feda0, obj=0x7fffffffe100) at linker.c:1296
    1296		Elf64_Sym *sym = symtab->data->d_buf;

Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
---
v1: https://lore.kernel.org/lkml/tencent_410B8166C55CD2AB64BDEA8E92204619180A@xxxxxx/
---
 tools/bpf/bpftool/gen.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 5a4d3240689e..506d205138db 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -1879,6 +1879,8 @@ static int do_object(int argc, char **argv)
 	struct bpf_linker *linker;
 	const char *output_file, *file;
 	int err = 0;
+	int argc_cpy = argc;
+	char **argv_cpy = argv;
 
 	if (!REQ_ARGS(2)) {
 		usage();
@@ -1887,6 +1889,14 @@ static int do_object(int argc, char **argv)
 
 	output_file = GET_ARG();
 
+	/* Ensure we don't overwrite any input file */
+	while (argc_cpy--) {
+		if (!strcmp(output_file, *argv_cpy++)) {
+			p_err("Input and output files cannot be the same");
+			goto out;
+		}
+	}
+
 	linker = bpf_linker__new(output_file, NULL);
 	if (!linker) {
 		p_err("failed to create BPF linker instance");
-- 
2.47.1