Date:   Wed, 19 Aug 2020 08:31:51 -0700
From:   Yonghong Song <yhs@...com>
To:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     Mark Wielaard <mjw@...hat.com>, Nick Clifton <nickc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        <netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next] tools/resolve_btfids: Fix sections with wrong
 alignment



On 8/19/20 2:23 AM, Jiri Olsa wrote:
> The data of compressed section should be aligned to 4
> (for 32bit) or 8 (for 64 bit) bytes.
> 
> The binutils ld sets sh_addralign to 1, which makes libelf
> fail with misaligned section error during the update as
> reported by Jesper:
> 
>     FAILED elf_update(WRITE): invalid section alignment
> 
> While waiting for ld fix, we can fix compressed sections
> sh_addralign value manually.
> 
> Adding warning in -vv mode when the fix is triggered:
> 
>    $ ./tools/bpf/resolve_btfids/resolve_btfids -vv vmlinux
>    ...
>    section(36) .comment, size 44, link 0, flags 30, type=1
>    section(37) .debug_aranges, size 45684, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 16, expected 8
>    section(38) .debug_info, size 129104957, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 1, expected 8
>    section(39) .debug_abbrev, size 1152583, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 1, expected 8
>    section(40) .debug_line, size 7374522, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 1, expected 8
>    section(41) .debug_frame, size 702463, link 0, flags 800, type=1
>    section(42) .debug_str, size 1017571, link 0, flags 830, type=1
>     - fixing wrong alignment sh_addralign 1, expected 8
>    section(43) .debug_loc, size 3019453, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 1, expected 8
>    section(44) .debug_ranges, size 1744583, link 0, flags 800, type=1
>     - fixing wrong alignment sh_addralign 16, expected 8
>    section(45) .symtab, size 2955888, link 46, flags 0, type=2
>    section(46) .strtab, size 2613072, link 0, flags 0, type=3
>    ...
>    update ok for vmlinux
> 
> Another workaround is to disable compressed debug info data
> CONFIG_DEBUG_INFO_COMPRESSED kernel option.

So CONFIG_DEBUG_INFO_COMPRESSED is required to reproduce the bug, right?

I turned on CONFIG_DEBUG_INFO_COMPRESSED in my config and got a bunch of
build failures.

ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize 
decompress status for section .debug_info
ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize 
decompress status for section .debug_info
ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize 
decompress status for section .debug_info
ld: drivers/crypto/virtio/virtio_crypto_algs.o: unable to initialize 
decompress status for section .debug_info
drivers/crypto/virtio/virtio_crypto_algs.o: file not recognized: File 
format not recognized

ld: net/llc/llc_core.o: unable to initialize decompress status for 
section .debug_info
ld: net/llc/llc_core.o: unable to initialize decompress status for 
section .debug_info
ld: net/llc/llc_core.o: unable to initialize decompress status for 
section .debug_info
ld: net/llc/llc_core.o: unable to initialize decompress status for 
section .debug_info
net/llc/llc_core.o: file not recognized: File format not recognized

...

The 'ld' in my system:

$ ld -V
GNU ld version 2.30-74.el8
   Supported emulations:
    elf_x86_64
    elf32_x86_64
    elf_i386
    elf_iamcu
    i386linux
    elf_l1om
    elf_k1om
    i386pep
    i386pe
$

Do you know what is the issue here?

> 
> Fixes: fbbb68de80a4 ("bpf: Add resolve_btfids tool to resolve BTF IDs in ELF object")
> Cc: Mark Wielaard <mjw@...hat.com>
> Cc: Nick Clifton <nickc@...hat.com>
> Reported-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>   tools/bpf/resolve_btfids/main.c | 36 +++++++++++++++++++++++++++++++++
>   1 file changed, 36 insertions(+)
> 
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 4d9ecb975862..0def0bb1f783 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -233,6 +233,39 @@ static struct btf_id *add_symbol(struct rb_root *root, char *name, size_t size)
>   	return btf_id__add(root, id, false);
>   }
>   
> +/*
> + * The data of compressed section should be aligned to 4
> + * (for 32bit) or 8 (for 64 bit) bytes. The binutils ld
> + * sets sh_addralign to 1, which makes libelf fail with
> + * misaligned section error during the update:
> + *    FAILED elf_update(WRITE): invalid section alignment
> + *
> + * While waiting for ld fix, we fix the compressed sections
> + * sh_addralign value manualy.
> + */
> +static int compressed_section_fix(Elf *elf, Elf_Scn *scn, GElf_Shdr *sh)
> +{
> +	int expected = gelf_getclass(elf) == ELFCLASS32 ? 4 : 8;
> +
> +	if (!(sh->sh_flags & SHF_COMPRESSED))
> +		return 0;
> +
> +	if (sh->sh_addralign == expected)
> +		return 0;
> +
> +	pr_debug2(" - fixing wrong alignment sh_addralign %u, expected %u\n",
> +		  sh->sh_addralign, expected);
> +
> +	sh->sh_addralign = expected;
> +
> +	if (gelf_update_shdr(scn, sh) == 0) {
> +		printf("FAILED cannot update section header: %s\n",
> +			elf_errmsg(-1));
> +		return -1;
> +	}
> +	return 0;
> +}
> +
>   static int elf_collect(struct object *obj)
>   {
>   	Elf_Scn *scn = NULL;
> @@ -309,6 +342,9 @@ static int elf_collect(struct object *obj)
>   			obj->efile.idlist_shndx = idx;
>   			obj->efile.idlist_addr  = sh.sh_addr;
>   		}
> +
> +		if (compressed_section_fix(elf, scn, &sh))
> +			return -1;
>   	}
>   
>   	return 0;
> 

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