Re: bpftool does not print full names with LLVM 17 and newer

[Ivan Babrou]

On Fri, May 17, 2024 at 4:33 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Fri, May 17, 2024 at 2:51 PM Ivan Babrou <ivan@xxxxxxxxxxxxxx> wrote:
> >
> > Hello,
> >
> > We recently bumped LLVM used for bpftool compilation from 15 to 18 and
> > our alerting system notified us about some unknown bpf programs. It
> > turns out, the names were truncated to 15 chars, whereas before they
> > were longer.
> >
> > After some investigation, I was able to see that the following code:
> >
> >     diff --git a/src/common.c b/src/common.c
> >     index 958e92a..ac38506 100644
> >     --- a/src/common.c
> >     +++ b/src/common.c
> >     @@ -435,7 +435,9 @@ void get_prog_full_name(const struct
> > bpf_prog_info *prog_info, int prog_fd,
> >         if (!prog_btf)
> >             goto copy_name;
> >
> >     +    printf("[0] finfo.type_id = %x\n", finfo.type_id);
> >         func_type = btf__type_by_id(prog_btf, finfo.type_id);
> >     +    printf("[1] finfo.type_id = %x\n", finfo.type_id);
> >         if (!func_type || !btf_is_func(func_type))
> >             goto copy_name;
> >
> > When ran under gdb, shows:
> >
> >     (gdb) b common.c:439
> >     Breakpoint 1 at 0x16859: file common.c, line 439.
> >
> >     (gdb) r
> >     3403: tracing  [0] finfo.type_id = 0
> >
> >     Breakpoint 1, get_prog_full_name (prog_info=0x7fffffffe160,
> > prog_fd=3, name_buff=0x7fffffffe030 "", buff_len=128) at common.c:439
> >     439        func_type = btf__type_by_id(prog_btf, finfo.type_id);
> >     (gdb) print finfo
> >     $1 = {insn_off = 0, type_id = 1547}
> >
> >
> > Notice that finfo.type_id is printed as zero, but in gdb it is in fact 1547.
> >
> > Disassembly difference looks like this:
> >
> >     -    8b 75 cc                 mov    -0x34(%rbp),%esi
> >     -    e8 47 8d 02 00           call   3f5b0 <btf__type_by_id>
> >     +    31 f6                    xor    %esi,%esi
> >     +    e8 a9 8c 02 00           call   3f510 <btf__type_by_id>
> >
> > This can be avoided if one removes "const" during finfo initialization:
> >
> >     const struct bpf_func_info finfo = {};
> >
> > This seems like a pretty annoying miscompilation, and hopefully
> > there's a way to make clang complain about this loudly, but that's
> > outside of my expertise. There might be other places like this that we
> > just haven't noticed yet.
> >
> > I can send a patch to fix this particular issue, but I'm hoping for a
> > more comprehensive approach from people who know better.
>
> Wow. Great catch. Please send a patch to fix bpftool and,
> I agree, llvm should be warning about such footgun,
> but the way ptr_to_u64() is written is probably silencing it.
> We probably should drop 'const' from it:
> static inline __u64 ptr_to_u64(const void *ptr)
>
> and maybe add a flavor of ptr_to_u64 with extra check
> that the arg doesn't have a const modifier.
> __builtin_types_compatible_p(typeof(ptr), void *)
> should do the trick.

In bpftool there's just two call sites that are unhappy if I remove
"const" in the arguments:

* this problematic one
* "GPL" literal passed

I'll send the patch to drop "const" from the struct initialization
today or tomorrow (it works great in our internal build), but I'll
leave the bigger change to you. There seem to be many places in libbpf
and I'm far from being a C expert to drive this change.

I managed to bisect clang to find the commit that introduced the change:

* https://github.com/llvm/llvm-project/commit/0b2d5b967d98

I also mentioned the commit author and they have some ideas about
UBSAN catching this (it doesn't in the current state):

* https://mastodon.ivan.computer/@mastodon/112465898861074834