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bpf: possibly avoid extra masking for narrower load in verifier

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Commit 31fd85816d ("bpf: permits narrower load from bpf program
context fields") permits narrower load for certain ctx fields.
The commit however will already generate a masking even if
the prog-specific ctx conversion produces the result with
narrower size.

For example, for __sk_buff->protocol, the ctx conversion
loads the data into register with 2-byte load.
A narrower 2-byte load should not generate masking.
For __sk_buff->vlan_present, the conversion function
set the result as either 0 or 1, essentially a byte.
The narrower 2-byte or 1-byte load should not generate masking.

To avoid unnecessary masking, prog-specific *_is_valid_access
now passes converted_op_size back to verifier, which indicates
the valid data width after perceived future conversion.
Based on this information, verifier is able to avoid
unnecessary marking.

Since we want more information back from prog-specific
*_is_valid_access checking, all of them are packed into
one data structure for more clarity.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Yonghong Song 2017-06-22 15:07:39 -07:00 • committed by David S. Miller
parent 72de46556f
commit 239946314e
5 changed files with 97 additions and 55 deletions
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3
include/linux/bpf_verifier.h
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@ -73,7 +73,8 @@ struct bpf_insn_aux_data {
enum bpf_reg_type ptr_type; /* pointer type for load/store insns */
struct bpf_map *map_ptr; /* pointer for call insn into lookup_elem */
};
int ctx_field_size; /* the ctx field size for load/store insns, maybe 0 */
int ctx_field_size; /* the ctx field size for load insn, maybe 0 */
int converted_op_size; /* the valid value width after perceived conversion */
};
#define MAX_USED_MAPS 64 /* max number of maps accessed by one eBPF program */