Commit 8819495a authored by Dave Thaler's avatar Dave Thaler Committed by Daniel Borkmann
Browse files

bpf, docs: Shift operations are defined to use a mask 



Update the documentation regarding shift operations to explain the
use of a mask, since otherwise shifting by a value out of range
(like negative) is undefined.

Signed-off-by: default avatarDave Thaler <dthaler@microsoft.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarYonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230509180845.1236-1-dthaler1968@googlemail.com
parent 2a36c26f

Documentation/bpf/instruction-set.rst

+6 −3
Original line number Diff line number Diff line
@@ -163,13 +163,13 @@ BPF_MUL 0x20 dst \*= src 
BPF_DIV   0x30   dst = (src != 0) ? (dst / src) : 0

BPF_OR    0x40   dst \|= src

BPF_AND   0x50   dst &= src

BPF_LSH   0x60   dst <<= src

BPF_RSH   0x70   dst >>= src

BPF_LSH   0x60   dst <<= (src & mask)

BPF_RSH   0x70   dst >>= (src & mask)

BPF_NEG   0x80   dst = ~src

BPF_MOD   0x90   dst = (src != 0) ? (dst % src) : dst

BPF_XOR   0xa0   dst ^= src

BPF_MOV   0xb0   dst = src

BPF_ARSH  0xc0   sign extending shift right

BPF_ARSH  0xc0   sign extending dst >>= (src & mask)

BPF_END   0xd0   byte swap operations (see `Byte swap instructions`_ below)

========  =====  ==========================================================
@@ -204,6 +204,9 @@ for ``BPF_ALU64``, 'imm' is first sign extended to 64 bits and the result 
interpreted as an unsigned 64-bit value. There are no instructions for

signed division or modulo.



Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)

for 32-bit operations.



Byte swap instructions

~~~~~~~~~~~~~~~~~~~~~~