linux-kernel - Re: [PATCH v2] fuse: uapi: use UAPI types

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Message-ID: <51731990-37fe-4821-9feb-7ee75829d3a0@bsbernd.com>
Date: Mon, 5 Jan 2026 09:50:30 +0100
From: Bernd Schubert <bernd@...ernd.com>
To: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Cc: Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2] fuse: uapi: use UAPI types



On 1/5/26 09:40, Thomas Weißschuh wrote:
> On Sat, Jan 03, 2026 at 01:44:49PM +0100, Bernd Schubert wrote:
>>
>>
>> On 1/2/26 23:27, Bernd Schubert wrote:
>>>
>>>
>>> On 12/30/25 13:10, Thomas Weißschuh wrote:
>>>> Using libc types and headers from the UAPI headers is problematic as it
>>>> introduces a dependency on a full C toolchain.
>>>>
>>>> Use the fixed-width integer types provided by the UAPI headers instead.
>>>> To keep compatibility with non-Linux platforms, add a stdint.h fallback.
>>>>
>>>> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
>>>> ---
>>>> Changes in v2:
>>>> - Fix structure member alignments
>>>> - Keep compatibility with non-Linux platforms
>>>> - Link to v1: https://lore.kernel.org/r/20251222-uapi-fuse-v1-1-85a61b87baa0@linutronix.de
>>>> ---
>>>>  include/uapi/linux/fuse.h | 626 +++++++++++++++++++++++-----------------------
>>>>  1 file changed, 319 insertions(+), 307 deletions(-)
>>>
>>> I tested this and it breaks libfuse compilation
>>>
>>> https://github.com/libfuse/libfuse/pull/1410
>>>
>>> Any chance you could test libfuse compilation for v3? Easiest way is to
>>> copy it to <libfuse>/include/fuse_kernel.h and then create PR. That
>>> includes a BSD test.
> 
> Ack.
> 
>>> libfuse3.so.3.19.0.p/fuse_uring.c.o -c
>>> ../../../home/runner/work/libfuse/libfuse/lib/fuse_uring.c
>>> ../../../home/runner/work/libfuse/libfuse/lib/fuse_uring.c:197:5: error:
>>> format specifies type 'unsigned long' but the argument has type '__u64'
>>> (aka 'unsigned long long') [-Werror,-Wformat]
>>>   196 |                 fuse_log(FUSE_LOG_DEBUG, "    unique: %" PRIu64
>>> ", result=%d\n",
>>>       |                                                       ~~~~~~~~~
>>>   197 |                          out->unique, ent_in_out->payload_sz);
>>>       |                          ^~~~~~~~~~~
>>> 1 error generated.
>>>
>>>
>>> I can certainly work it around in libfuse by adding a cast, IMHO,
>>> PRIu64 is the right format.
> 
> PRIu64 is indeed the right format for uint64_t. Unfortunately not necessarily
> for __u64. As the vast majority of the UAPI headers to use the UAPI types,
> adding a cast in this case is already necessary for most UAPI users.
> 
>> I think what would work is the attached version. Short interesting part
>>
>> #if defined(__KERNEL__)
>> #include <linux/types.h>
>> typedef __u8	fuse_u8;
>> typedef __u16	fuse_u16;
>> typedef __u32	fuse_u32;
>> typedef __u64	fuse_u64;
>> typedef __s8	fuse_s8;
>> typedef __s16	fuse_s16;
>> typedef __s32	fuse_s32;
>> typedef __s64	fuse_s64;
>> #else
>> #include <stdint.h>
>> typedef uint8_t		fuse_u8;
>> typedef uint16_t	fuse_u16;
>> typedef uint32_t	fuse_u32;
>> typedef uint64_t	fuse_u64;
>> typedef int8_t		fuse_s8;
>> typedef int16_t		fuse_s16;
>> typedef int32_t		fuse_s32;
>> typedef int64_t		fuse_s64;
>> #endif
> 
> Unfortunately this is equivalent to the status quo.
> It contains a dependency on the libc header stdint.h when used from userspace.
> 
> IMO the best way forward is to use the v2 patch and add a cast in fuse_uring.c.

libfuse is easy, but libfuse is just one library that might use/copy the
header. If libfuse breaks the others might as well.
Maybe you could explain your issue more detailed? I.e. how are you using
this include exactly?


Thanks,
Bernd