linux-kernel - Re: [PATCH 04/44] io_uring/net: Change some dubious min

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Message-ID: <20251120154817.0160eeac@pumpkin>
Date: Thu, 20 Nov 2025 15:48:17 +0000
From: David Laight <david.laight.linux@...il.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [PATCH 04/44] io_uring/net: Change some dubious min_t()

On Thu, 20 Nov 2025 07:48:58 -0700
Jens Axboe <axboe@...nel.dk> wrote:

> On 11/19/25 3:41 PM, david.laight.linux@...il.com wrote:
> > From: David Laight <david.laight.linux@...il.com>
> > 
> > Since iov_len is 'unsigned long' it is possible that the cast
> > to 'int' will change the value of min_t(int, iov[nbufs].iov_len, ret).
> > Use a plain min() and change the loop bottom to while (ret > 0) so that
> > the compiler knows 'ret' is always positive.
> > 
> > Also change min_t(int, sel->val, sr->mshot_total_len) to a simple min()
> > since sel->val is also long and subject to possible trunctation.
> > 
> > It might be that other checks stop these being problems, but they are
> > picked up by some compile-time tests for min_t() truncating values.  
> 
> Fails with clang-21:
> 
> io_uring/net.c:855:26: error: call to '__compiletime_assert_2006' declared with 'error' attribute: min(sel->val, sr->mshot_total_len) signedness error
>   855 |                 sr->mshot_total_len -= min(sel->val, sr->mshot_total_len);

I'll take a look, I normally use gcc but there must be something subtle going on.
Actually which architecture?
I only tested x86-64.

	David

>       |                                        ^
> ./include/linux/minmax.h:105:19: note: expanded from macro 'min'
>   105 | #define min(x, y)       __careful_cmp(min, x, y)
>       |                         ^
> ./include/linux/minmax.h:98:2: note: expanded from macro '__careful_cmp'
>    98 |         __careful_cmp_once(op, x, y, __UNIQUE_ID(x_), __UNIQUE_ID(y_))
>       |         ^
> ./include/linux/minmax.h:93:2: note: expanded from macro '__careful_cmp_once'
>    93 |         BUILD_BUG_ON_MSG(!__types_ok(ux, uy),           \
>       |         ^
> note: (skipping 2 expansions in backtrace; use -fmacro-backtrace-limit=0 to see all)
> ././include/linux/compiler_types.h:590:2: note: expanded from macro '_compiletime_assert'
>   590 |         __compiletime_assert(condition, msg, prefix, suffix)
>       |         ^
> ././include/linux/compiler_types.h:583:4: note: expanded from macro '__compiletime_assert'
>   583 |                         prefix ## suffix();                             \
>       |                         ^
> <scratch space>:319:1: note: expanded from here
>   319 | __compiletime_assert_2006
>       | ^
>