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

[Rong Tao <rtoax@foxmail.com>]

On 12/5/24 18:40, Quentin Monnet wrote:
> On 05/12/2024 09:08, 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.
>>
>>      $ sudo ./bpftool gen object prog.o prog.o
>
> (No sudo required to generate object files)
>
>
>>      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.");
>
> Nits: lowercase for "output", and "cannot" rather than "couldn't"; also
> bpftool doesn't use periods at the end of error messages:
>
>      p_err("Input and output files cannot be the 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);
>
> Good catch, thank you for this!
>
> I've got one concern though, while your patch addresses the segfault it
> doesn't prevent the user from overwriting the input file. Could we
> instead move the check above the call to bpf_linker__new(...), please?
> Something like this:
>
> 	int argc_cpy = argc;
> 	char **argv_cpy = argv;
>
> 	[...]
> 	output_file = GET_ARG();
>
> 	/* Ensure we don't overwrite any input file */
> 	while (argc_cpy--) {
> 		if (!strcmp(output_file, *argv_cpy++)) {
> 			p_err("...");
> 			goto out;
> 		}
> 	}

Thanks a lot, I just submit v2, please review!!

Rong Tao

>
> 	linker = ...
>
> Thanks,
> Quentin
>
> pw-bot: cr