netdev - Re: [PATCH bpf 1/3] bpf: don't assume build-id length is always 20 bytes

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Message-ID: <0FC05078-4DF7-453E-8F36-7C0F5AF81F69@fb.com>
Date:   Wed, 16 Jan 2019 17:45:30 +0000
From:   Song Liu <songliubraving@...com>
To:     Stanislav Fomichev <sdf@...gle.com>
CC:     Netdev <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: Re: [PATCH bpf 1/3] bpf: don't assume build-id length is always 20
 bytes



> On Jan 15, 2019, at 2:54 PM, Stanislav Fomichev <sdf@...gle.com> wrote:
> 
> Build-id length is not fixed to 20, it can be (`man ld` /--build-id):
>  * 128-bit (uuid)
>  * 160-bit (sha1)
>  * any length specified in ld --build-id=0xhexstring
> 
> To fix the issue of missing BPF_STACK_BUILD_ID_VALID for shorter build-ids,
> assume that build-id is somewhere in the range of 1 .. 20.
> Set the remaining bytes to zero.
> 
> Fixes: 615755a77b24 ("bpf: extend stackmap to save binary_build_id+offset instead of address")
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
> kernel/bpf/stackmap.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index d9e2483669d0..8975d1768dcb 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -180,11 +180,15 @@ static inline int stack_map_parse_build_id(void *page_addr,
> 
> 		if (nhdr->n_type == BPF_BUILD_ID &&
> 		    nhdr->n_namesz == sizeof("GNU") &&
> -		    nhdr->n_descsz == BPF_BUILD_ID_SIZE) {
> +		    nhdr->n_descsz > 0 &&
> +		    nhdr->n_descsz <= BPF_BUILD_ID_SIZE) {
> +			__u32 len = min_t(__u32,
> +					  BPF_BUILD_ID_SIZE, nhdr->n_descsz);

Given the check above, we only need len = nhdr->n_descsz, right?

Other than this, 

Acked-by: Song Liu <songliubraving@...com>

Thanks for the fix!

> 			memcpy(build_id,
> 			       note_start + note_offs +
> 			       ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr),
> -			       BPF_BUILD_ID_SIZE);
> +			       len);
> +			memset(build_id + len, 0, BPF_BUILD_ID_SIZE - len);
> 			return 0;
> 		}
> 		new_offs = note_offs + sizeof(Elf32_Nhdr) +
> -- 
> 2.20.1.97.g81188d93c3-goog
>