Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Andrii Nakryiko
2021-02-09T06:10:38Z

On Mon, Feb 8, 2021 at 9:23 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Mon, Feb 08, 2021 at 08:45:43PM -0800, Andrii Nakryiko wrote:
> > On Mon, Feb 8, 2021 at 7:44 PM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > Hi all,
> > >
> > > Recently, an issue with CONFIG_DEBUG_INFO_BTF was reported for arm64:
> > > https://groups.google.com/g/clang-built-linux/c/de_mNh23FOc/m/E7cu5BwbBAAJ
> > >
> > > $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
> > > LLVM=1 O=build/aarch64 defconfig
> > >
> > > $ scripts/config \
> > > --file build/aarch64/.config \
> > > -e BPF_SYSCALL \
> > > -e DEBUG_INFO_BTF \
> > > -e FTRACE \
> > > -e FUNCTION_TRACER
> > >
> > > $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- \
> > > LLVM=1 O=build/aarch64 olddefconfig all
> > > ...
> > > FAILED unresolved symbol vfs_truncate
> > > ...
> > >
> > > My bisect landed on commit 6e22ab9da793 ("bpf: Add d_path helper")
> > > although that seems obvious given that is what introduced
> > > BTF_ID(func, vfs_truncate).
> > >
> > > I am using the latest pahole v1.20 and LLVM is at
> > > https://github.com/llvm/llvm-project/commit/14da287e18846ea86e45b421dc47f78ecc5aa7cb
> > > although I can reproduce back to LLVM 10.0.1, which is the earliest
> > > version that the kernel supports. I am very unfamiliar with BPF so I
> > > have no idea what is going wrong here. Is this a known issue?
> > >
> >
> > I'll skip the reproduction games this time and will just request the
> > vmlinux image. Please upload somewhere so that we can look at DWARF
> > and see what's going on. Thanks.
> >
>
> Sure thing, let me know if this works. I uploaded in two places to make
> it easier to grab:
>
> zstd compressed:
> https://github.com/nathanchance/bug-files/blob/3b2873751e29311e084ae2c71604a1963f5e1a48/btf-aarch64/vmlinux.zst
>

Thanks. I clearly see at least one instance of seemingly well-formed
vfs_truncate DWARF declaration. Also there is a proper ELF symbol for
it. Which means it should have been generated in BTF, but it doesn't
appear to be, so it does seem like a pahole bug. I (or someone else
before me) will continue tomorrow.

$ llvm-dwarfdump vmlinux
...

0x00052e6f: DW_TAG_subprogram
DW_AT_name ("vfs_truncate")
DW_AT_decl_file
("/home/nathan/cbl/src/linux/include/linux/fs.h")
DW_AT_decl_line (2520)
DW_AT_prototyped (true)
DW_AT_type (0x000452cb "long int")
DW_AT_declaration (true)
DW_AT_external (true)

0x00052e7b: DW_TAG_formal_parameter
DW_AT_type (0x00045fc6 "const path*")

0x00052e80: DW_TAG_formal_parameter
DW_AT_type (0x00045213 "long long int")

...

$ llvm-readelf -s vmlinux | rg vfs_truncate
15013: ffff800011c22418 4 OBJECT LOCAL DEFAULT 24
__BTF_ID__func__vfs_truncate__609
22531: ffff80001189fe0d 0 NOTYPE LOCAL DEFAULT 17
__kstrtab_vfs_truncate
22532: ffff8000118a985b 0 NOTYPE LOCAL DEFAULT 17
__kstrtabns_vfs_truncate
22534: ffff800011873b7c 0 NOTYPE LOCAL DEFAULT 8
__ksymtab_vfs_truncate
176099: ffff80001031f430 360 FUNC GLOBAL DEFAULT 2 vfs_truncate

$ bpftool btf dump file vmlinux | rg vfs_truncate
<nothing>

> uncompressed:
> https://1drv.ms/u/s!AsQNYeB-IEbqjQiUOspbEdXx49o7?e=ipA9Hv
>
> Cheers,
> Nathan