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

[Quentin Monnet <qmo@kernel.org>]

On 05/12/2024 11:10, Rong Tao wrote:
> 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.
> 
>     $ 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>
> ---
> v1: https://lore.kernel.org/lkml/tencent_410B8166C55CD2AB64BDEA8E92204619180A@qq.com/
> ---
>  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;


Oops sorry, argc_cpy and argv_cpy need to be initialised _after_ the
call to GET_ARG() below, otherwise we always start comparing output_file
with itself. Please test this code on your side, too :)

pw-bot: cr


>  
>  	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");