Re: [PATCH bpf-next v3] bpftool: Fix gen object segfault

[Andrii Nakryiko]

On Thu, Dec 5, 2024 at 4:22 AM Quentin Monnet <qmo@xxxxxxxxxx> wrote:
>
> On 05/12/2024 12:09, Rong Tao wrote:
> > 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>
>
> Tested-by: Quentin Monnet <qmo@xxxxxxxxxx>
> Reviewed-by: Quentin Monnet <qmo@xxxxxxxxxx>

Isn't this papering over a deeper underlying issue? Why do we get
SIGSEGV inside the linker at all instead of just erroring out?
Comparison based on file path isn't a reliable way to check if input
and output are both the same file, so this fixes the most obvious
case, but not the actual issue.

>
> Thank you!