Re: [PATCH bpf] bpf/btf: Accept function names that contain dots

[Florent Revest]

On Mon, Jun 19, 2023 at 8:17 PM Yonghong Song <yhs@xxxxxxxx> wrote:
>
>
>
> On 6/19/23 7:03 AM, Florent Revest wrote:
> > On Fri, Jun 16, 2023 at 6:57 PM Andrii Nakryiko
> > <andrii.nakryiko@xxxxxxxxx> wrote:
> >>
> >> On Thu, Jun 15, 2023 at 7:56 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
> >>>
> >>> When building a kernel with LLVM=1, LLVM_IAS=0 and CONFIG_KASAN=y, LLVM
> >>> leaves DWARF tags for the "asan.module_ctor" & co symbols. In turn,
> >>> pahole creates BTF_KIND_FUNC entries for these and this makes the BTF
> >>> metadata validation fail because they contain a dot.
> >>>
> >>> In a dramatic turn of event, this BTF verification failure can cause
> >>> the netfilter_bpf initialization to fail, causing netfilter_core to
> >>> free the netfilter_helper hashmap and netfilter_ftp to trigger a
> >>> use-after-free. The risk of u-a-f in netfilter will be addressed
> >>> separately but the existence of "asan.module_ctor" debug info under some
> >>> build conditions sounds like a good enough reason to accept functions
> >>> that contain dots in BTF.
> >>
> >> I don't see much harm in allowing dots. There are also all those .isra
> >> and other modifications to functions that we currently don't have in
> >> BTF, but with the discussions about recording function addrs we might
> >> eventually have those as well. So:
> >>
> >> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> >
> > Thanks Andrii! :)
> >
> >>> Cc: stable@xxxxxxxxxxxxxxx
> >>> Fixes: 1dc92851849c ("bpf: kernel side support for BTF Var and DataSec")
> >
> > So do you think these trailers should be kept ? I suppose we can
> > either see this as a "new feature" to accommodate .isra that should go
> > through bpf-next or as a bug fix that goes through bpf and gets
> > backported to stable (without this, BTF wouldn't work on old kernels
> > built under a new clang and with LLVM_IAS=0 and CONFIG_KASAN=y so this
> > sounds like a legitimate bug fix to me, I just wanted to double check)
>
> How many people really build the kernel with
>     LLVM=1 LLVM_IAS=0
> which uses clang compiler ans gcc 'as'.
> I think distro most likely won't do this if they intend to
> build the kernel with clang.

I tend to agree with you

> Note that
>     LLVM=1
> implies to use both clang compiler and clang assembler.

However, this is only true since:
f12b034afeb3 ("scripts/Makefile.clang: default to LLVM_IAS=1")

5.10 stable for example does not have that commit and LLVM_IAS=0 is
still the default there. (actually that's how I stumbled upon this: by
building a 5.10 LTS and then finding a way to reproduce it upstream by
disabling LLVM_IAS)

> Using clang17 and 'LLVM=1 LLVM_IAS=0', with latest bpf-next,
> I actually hit some build errors like:
>
> /tmp/video-bios-59fa52.s: Assembler messages:
> /tmp/video-bios-59fa52.s:4: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:4: Error: file number less than one
> /tmp/video-bios-59fa52.s:5: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:6: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:7: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:8: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:9: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:10: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/video-bios-59fa52.s:68: Error: junk at end of line, first
> unrecognized character is `"'
> clang: error: assembler command failed with exit code 1 (use -v to see
> invocation)
> make[4]: *** [/home/yhs/work/bpf-next/scripts/Makefile.build:252:
> arch/x86/realmode/rm/video-bios.o] Error 1
> make[4]: *** Waiting for unfinished jobs....
> /tmp/wakemain-88777c.s: Assembler messages:
> /tmp/wakemain-88777c.s:4: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:4: Error: file number less than one
> /tmp/wakemain-88777c.s:5: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:6: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:7: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:8: Error: junk at end of line, first unrecognized
> character is `"'
> /tmp/wakemain-88777c.s:81: Error: junk at end of line, first
> unrecognized character is `"'
> /tmp/wakemain-88777c.s:312: Error: junk at end of line, first
> unrecognized character is `"'
> clang: error: assembler command failed with exit code 1 (use -v to see
> invocation)
>
> Potentially because of my local gnu assembler 2.30-120.el8 won't work
> with some syntax generated by clang. Mixing clang compiler and arbitrary
> gnu assembler are not a good idea (see the above example). It might
> work with close-to-latest gnu assembler.

I did not hit this bug with clang 17 and bpf-next so it's probably an
incompatibility with that gnu assembler indeed.

> To support function name like '<fname>.isra', some llvm work will be
> needed, and it may take some time.
>
> So in my opinion, this patch is NOT a bug fix. It won't affect distro.
> Whether we should backport to the old kernel, I am not sure whether it
> is absolutely necessary as casual build can always remove LLVM_IAS=0 or
> hack the kernel source itself.

If you think it's not worth backporting to 5.10 (where LLVM_IAS=0 is
the default) then I could drop these trailers and send it to bpf-next
with a different justification. Either way is fine by me.