Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Andrii Nakryiko
2021-02-09T06:57:50Z

On Mon, Feb 8, 2021 at 10:13 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Feb 8, 2021 at 10:09 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > 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")
> >
> > ...
> >
>
> ... and here's the *only* other one (not marked as declaration, but I
> thought we already handle that, Jiri?):
>
> 0x01d0da35: DW_TAG_subprogram
> DW_AT_low_pc (0xffff80001031f430)
> DW_AT_high_pc (0xffff80001031f598)
> DW_AT_frame_base (DW_OP_reg29)
> DW_AT_GNU_all_call_sites (true)
> DW_AT_name ("vfs_truncate")
> DW_AT_decl_file ("/home/nathan/cbl/src/linux/fs/open.c")
> DW_AT_decl_line (69)
> DW_AT_prototyped (true)
> DW_AT_type (0x01cfdfe4 "long int")
> DW_AT_external (true)
>

Ok, the problem appears to be not in DWARF, but in mcount_loc data.
vfs_truncate's address is not recorded as ftrace-attachable, and thus
pahole ignores it. I don't know why this happens and it's quite
strange, given vfs_truncate is just a normal global function.

I'd like to understand this issue before we try to fix it, but there
is at least one improvement we can make: pahole should check ftrace
addresses only for static functions, not the global ones (global ones
should be always attachable, unless they are special, e.g., notrace
and stuff). We can easily check that by looking at the corresponding
symbol. But I'd like to verify that vfs_truncate is ftrace-attachable
for that particular kernel. For that we'll need Nathan's cooperation,
unless someone else can build an arm64 kernel with the same problem
and check.

>
> > $ 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