Commit 31ce8c4a authored by Arthur Fabre's avatar Arthur Fabre Committed by Daniel Borkmann
Browse files

bpf, doc: Document Jump X addressing mode 



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: default avatarArthur Fabre <arthur@arthurfabre.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent 1bc38b8f

Documentation/networking/filter.txt

+16 −14
Original line number Original line Diff line number Diff line
@@ -203,11 +203,11 @@ opcodes as defined in linux/filter.h stand for: 




  Instruction      Addressing mode      Description

  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

  ldi              4                    Load word into A

  ldh              1, 2                 Load half-word into A

  ldh              1, 2                 Load half-word into A

  ldb              1, 2                 Load byte 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

  ldxi             4                    Load word into X

  ldxb             5                    Load byte 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

  jmp              6                    Jump to label

  ja               6                    Jump to label

  ja               6                    Jump to label

  jeq              7, 8                 Jump on A == k

  jeq              7, 8, 9, 10          Jump on A == <x>

  jneq             8                    Jump on A != k

  jneq             9, 10                Jump on A != <x>

  jne              8                    Jump on A != k

  jne              9, 10                Jump on A != <x>

  jlt              8                    Jump on A <  k

  jlt              9, 10                Jump on A <  <x>

  jle              8                    Jump on A <= k

  jle              9, 10                Jump on A <= <x>

  jgt              7, 8                 Jump on A >  k

  jgt              7, 8, 9, 10          Jump on A >  <x>

  jge              7, 8                 Jump on A >= k

  jge              7, 8, 9, 10          Jump on A >= <x>

  jset             7, 8                 Jump on A &  k

  jset             7, 8, 9, 10          Jump on A &  <x>





  add              0, 4                 A + <x>

  add              0, 4                 A + <x>

  sub              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

  tax                                   Copy A into X

  txa                                   Copy X into A

  txa                                   Copy X into A





  ret              4, 9                 Return

  ret              4, 11                Return





The next table shows addressing formats from the 2nd column:

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

   5               4*([k]&0xf)          Lower nibble * 4 at byte offset k in the packet

   6               L                    Jump label L

   6               L                    Jump label L

   7               #k,Lt,Lf             Jump to Lt if true, otherwise jump to Lf

   7               #k,Lt,Lf             Jump to Lt if true, otherwise jump to Lf

   8               #k,Lt                Jump to Lt if predicate is true

   8               x/%x,Lt,Lf           Jump to Lt if true, otherwise jump to Lf

   9               a/%a                 Accumulator A

   9               #k,Lt                Jump to Lt if predicate is true

  10               extension            BPF extension

  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

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

with the class of load instructions by "overloading" the k argument with

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