Re: FAILED unresolved symbol vfs_truncate on arm64 with LLVM

Nathan Chancellor
2021-02-09T07:50:25Z

On Mon, Feb 08, 2021 at 10:56:36PM -0800, Andrii Nakryiko wrote:
> 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.

Sure, just let me know what I need to do and I can do it!

Cheers,
Nathan

> >
> > > $ 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
>
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