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

[Rong Tao <rtoax@foxmail.com>]

From: Rong Tao <rongtao@cestc.cn>

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.

    $ sudo ./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@cestc.cn>
---
 tools/bpf/bpftool/gen.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 5a4d3240689e..4cd135726758 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -1896,6 +1896,11 @@ static int do_object(int argc, char **argv)
 	while (argc) {
 		file = GET_ARG();
 
+		if (!strcmp(file, output_file)) {
+			p_err("Input/Output file couldn't be same.");
+			goto out;
+		}
+
 		err = bpf_linker__add_file(linker, file, NULL);
 		if (err) {
 			p_err("failed to link '%s': %s (%d)", file, strerror(errno), errno);
-- 
2.47.1