netdev - [PATCH net 2/2] bpf: fix checking xdp_adjust

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Date:   Mon, 17 Apr 2017 03:12:07 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     davem@...emloft.net
Cc:     alexei.starovoitov@...il.com, kubakici@...pl,
        netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>
Subject: [PATCH net 2/2] bpf: fix checking xdp_adjust_head on tail calls

Commit 17bedab27231 ("bpf: xdp: Allow head adjustment in XDP prog")
added the xdp_adjust_head bit to the BPF prog in order to tell drivers
that the program that is to be attached requires support for the XDP
bpf_xdp_adjust_head() helper such that drivers not supporting this
helper can reject the program. There are also drivers that do support
the helper, but need to check for xdp_adjust_head bit in order to move
packet metadata prepended by the firmware away for making headroom.

For these cases, the current check for xdp_adjust_head bit is insufficient
since there can be cases where the program itself does not use the
bpf_xdp_adjust_head() helper, but tail calls into another program that
uses bpf_xdp_adjust_head(). As such, the xdp_adjust_head bit is still
set to 0. Since the first program has no control over which program it
calls into, we need to assume that bpf_xdp_adjust_head() helper is used
upon tail calls. Thus, for the very same reasons in cb_access, set the
xdp_adjust_head bit to 1 when the main program uses tail calls.

Fixes: 17bedab27231 ("bpf: xdp: Allow head adjustment in XDP prog")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Cc: Martin KaFai Lau <kafai@...com>
---
 kernel/bpf/syscall.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index ee5c969..821f9e8 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -623,6 +623,7 @@ static void fixup_bpf_calls(struct bpf_prog *prog)
 				 * in the program array.
 				 */
 				prog->cb_access = 1;
+				prog->xdp_adjust_head = 1;

 				/* mark bpf_tail_call as different opcode
 				 * to avoid conditional branch in
-- 
1.9.3