Subject: Re: [PATCH bpf-next 1/3] bpf: Treat bpf_sk_lookup remote_port as a 2-byte field

From: Jakub Sitnicki <[email-masked]>
Date: Fri, 18 Mar 2022 16:22:57 +0100

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 <[email-masked]>
>> ---
>>  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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