netdev - Re: [PATCH v1 bpf-next 3/5] bpf: udp: Propagate ENOMEM up from bpf_iter_udp

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Message-ID: <20250410202718.7676-1-kuniyu@amazon.com>
Date: Thu, 10 Apr 2025 13:27:12 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <jordan@...fe.io>
CC: <aditi.ghag@...valent.com>, <bpf@...r.kernel.org>, <daniel@...earbox.net>,
	<kuniyu@...zon.com>, <martin.lau@...ux.dev>, <netdev@...r.kernel.org>,
	<willemdebruijn.kernel@...il.com>
Subject: Re: [PATCH v1 bpf-next 3/5] bpf: udp: Propagate ENOMEM up from bpf_iter_udp_batch

From: Jordan Rife <jordan@...fe.io>
Date: Wed,  9 Apr 2025 11:22:32 -0700
> Stop iteration if the current bucket can't be contained in a single
> batch to avoid choosing between skipping or repeating sockets. In cases
> where none of the saved cookies can be found in the current bucket and
> the batch isn't big enough to contain all the sockets in the bucket,
> there are really only two choices, neither of which is desirable:
> 
> 1. Start from the beginning, assuming we haven't seen any sockets in the
>    current bucket, in which case we might repeat a socket we've already
>    seen.
> 2. Go to the next bucket to avoid repeating a socket we may have already
>    seen, in which case we may accidentally skip a socket that we didn't
>    yet visit.
> 
> To avoid this tradeoff, enforce the invariant that the batch always
> contains a full snapshot of the bucket from last time by returning
> -ENOMEM if bpf_iter_udp_realloc_batch() can't grab enough memory to fit
> all sockets in the current bucket.
> 
> To test this code path, I forced bpf_iter_udp_realloc_batch() to return
> -ENOMEM when called from within bpf_iter_udp_batch() and observed that
> read() fails in userspace with errno set to ENOMEM. Otherwise, it's a
> bit hard to test this scenario.
> 
> Link: https://lore.kernel.org/bpf/CABi4-ogUtMrH8-NVB6W8Xg_F_KDLq=yy-yu-tKr2udXE2Mu1Lg@mail.gmail.com/
> 
> Signed-off-by: Jordan Rife <jordan@...fe.io>

I felt this patch is patch 2 but have no strong preference.

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>