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

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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: Dave Thaler <dthaler@microsoft.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20230509180845.1236-1-dthaler1968@googlemail.com
This commit is contained in:
Dave Thaler 2023-05-09 18:08:45 +00:00 committed by Daniel Borkmann
parent 2a36c26fe3
commit 8819495a75
1 changed files with 6 additions and 3 deletions
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Documentation/bpf/instruction-set.rst
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@ -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
~~~~~~~~~~~~~~~~~~~~~~