On Thu, Mar 17, 2022 at 06:22 PM -07, Martin KaFai Lau wrote:
> On Thu, Mar 17, 2022 at 05:58:24PM +0100, Jakub Sitnicki wrote:
>> In commit 9a69e2b385f4 ("bpf: Make remote_port field in struct
>> bpf_sk_lookup 16-bit wide") the remote_port field has been split up and
>> re-declared from u32 to be16.
>>
>> However, the accompanying changes to the context access converter have not
>> been well thought through when it comes big-endian platforms.
>>
>> Today 2-byte wide loads from offsetof(struct bpf_sk_lookup, remote_port)
>> are handled as narrow loads from a 4-byte wide field.
>>
>> This by itself is not enough to create a problem, but when we combine
>>
>> 1. 32-bit wide access to ->remote_port backed by a 16-wide wide load, with
>> 2. inherent difference between litte- and big-endian in how narrow loads
>> need have to be handled (see bpf_ctx_narrow_access_offset),
>>
>> we get inconsistent results for a 2-byte loads from &ctx->remote_port on LE
>> and BE architectures. This in turn makes BPF C code for the common case of
>> 2-byte load from ctx->remote_port not portable.
>>
>> To rectify it, inform the context access converter that remote_port is
>> 2-byte wide field, and only 1-byte loads need to be treated as narrow
>> loads.
>>
>> At the same time, we special-case the 4-byte load from &ctx->remote_port to
>> continue handling it the same way as do today, in order to keep the
>> existing BPF programs working.
>>
>> Fixes: 9a69e2b385f4 ("bpf: Make remote_port field in struct bpf_sk_lookup 16-bit wide")
>> Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
>> ---
>> net/core/filter.c | 20 ++++++++++++++++++--
>> 1 file changed, 18 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 03655f2074ae..9b1e453baf6d 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -10989,13 +10989,24 @@ static bool sk_lookup_is_valid_access(int off, int size,
>> case bpf_ctx_range(struct bpf_sk_lookup, local_ip4):
>> case bpf_ctx_range_till(struct bpf_sk_lookup, remote_ip6[0], remote_ip6[3]):
>> case bpf_ctx_range_till(struct bpf_sk_lookup, local_ip6[0], local_ip6[3]):
>> - case offsetof(struct bpf_sk_lookup, remote_port) ...
>> - offsetof(struct bpf_sk_lookup, local_ip4) - 1:
>> case bpf_ctx_range(struct bpf_sk_lookup, local_port):
>> case bpf_ctx_range(struct bpf_sk_lookup, ingress_ifindex):
>> bpf_ctx_record_field_size(info, sizeof(__u32));
>> return bpf_ctx_narrow_access_ok(off, size, sizeof(__u32));
>>
>> + case bpf_ctx_range(struct bpf_sk_lookup, remote_port):
>> + /* Allow 4-byte access to 2-byte field for backward compatibility */
>> + if (size == sizeof(__u32))
>> + return off == offsetof(struct bpf_sk_lookup, remote_port);
> nit. The bad "off" value should have been rejected earlier in the
> "if (off % size != 0)" check?
Good catch. That is always true. I will respin.
Thanks for reviewing the patch sets.
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