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bpf, doc: Document Jump X addressing mode

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bpf_asm and the other classic BPF tools support jump conditions
comparing register A to register X, in addition to comparing
register A with constant K.

Only the latter was documented in filter.txt, add two new addressing
modes that describe the former.

Signed-off-by: Arthur Fabre <arthur@arthurfabre.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Arthur Fabre 2018-10-07 09:45:19 +01:00 committed by Daniel Borkmann
parent 1bc38b8ff6
commit 31ce8c4a1a
1 changed files with 16 additions and 14 deletions
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30
Documentation/networking/filter.txt
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@ -203,11 +203,11 @@ opcodes as defined in linux/filter.h stand for:
Instruction Addressing mode Description
ld 1, 2, 3, 4, 10 Load word into A
ld 1, 2, 3, 4, 12 Load word into A
ldi 4 Load word into A
ldh 1, 2 Load half-word into A
ldb 1, 2 Load byte into A
ldx 3, 4, 5, 10 Load word into X
ldx 3, 4, 5, 12 Load word into X
ldxi 4 Load word into X
ldxb 5 Load byte into X
@ -216,14 +216,14 @@ opcodes as defined in linux/filter.h stand for:
jmp 6 Jump to label
ja 6 Jump to label
jeq 7, 8 Jump on A == k
jneq 8 Jump on A != k
jne 8 Jump on A != k
jlt 8 Jump on A < k
jle 8 Jump on A <= k
jgt 7, 8 Jump on A > k
jge 7, 8 Jump on A >= k
jset 7, 8 Jump on A & k
jeq 7, 8, 9, 10 Jump on A == <x>
jneq 9, 10 Jump on A != <x>
jne 9, 10 Jump on A != <x>
jlt 9, 10 Jump on A < <x>
jle 9, 10 Jump on A <= <x>
jgt 7, 8, 9, 10 Jump on A > <x>
jge 7, 8, 9, 10 Jump on A >= <x>
jset 7, 8, 9, 10 Jump on A & <x>
add 0, 4 A + <x>
sub 0, 4 A - <x>
@ -240,7 +240,7 @@ opcodes as defined in linux/filter.h stand for:
tax Copy A into X
txa Copy X into A
ret 4, 9 Return
ret 4, 11 Return
The next table shows addressing formats from the 2nd column:
@ -254,9 +254,11 @@ The next table shows addressing formats from the 2nd column:
5 4*([k]&0xf) Lower nibble * 4 at byte offset k in the packet
6 L Jump label L
7 #k,Lt,Lf Jump to Lt if true, otherwise jump to Lf
8 #k,Lt Jump to Lt if predicate is true
9 a/%a Accumulator A
10 extension BPF extension
8 x/%x,Lt,Lf Jump to Lt if true, otherwise jump to Lf
9 #k,Lt Jump to Lt if predicate is true
10 x/%x,Lt Jump to Lt if predicate is true
11 a/%a Accumulator A
12 extension BPF extension
The Linux kernel also has a couple of BPF extensions that are used along
with the class of load instructions by "overloading" the k argument with