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

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Message-ID: <20260109085917-e316ce57-5e78-4827-96d7-4a48a68aa752@linutronix.de>
Date: Fri, 9 Jan 2026 09:11:28 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Bernd Schubert <bernd@...ernd.com>
Cc: Arnd Bergmann <arnd@...db.de>, Miklos Szeredi <miklos@...redi.hu>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fuse: uapi: use UAPI types

On Thu, Jan 08, 2026 at 11:12:29PM +0100, Bernd Schubert wrote:
> 
> 
> On 1/5/26 13:09, Arnd Bergmann wrote:
> > On Mon, Jan 5, 2026, at 09:50, Bernd Schubert wrote:
> >> On 1/5/26 09:40, Thomas Weißschuh wrote:
> >>> On Sat, Jan 03, 2026 at 01:44:49PM +0100, Bernd Schubert wrote:
> >>>
> >>>>> 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.
> > 
> > Which target did the warning show up on? I would expect the patch
> > to not have changed anything for BSD, since not defining __linux__
> > makes it use the stdint types after all.
> > 
> > On alpha/mips/powerpc, the default is to use 'unsigned long' unless
> > the __SANE_USERSPACE_TYPES__ macro is defined before linux/types.h
> > gets included, and we may be able to do the same on other
> > architectures as well for consistency. All the other 64-bit
> > architectures (x86-64, arm64, riscv64, s390x, parisc64, sparc64)
> > only provide the ll64 types from uapi but seem to use the l64
> > version both in gcc's predefined types and in glibc.
> > 
> >>>> 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.
> > 
> > I don't think we'll find a solution that won't break somewhere,
> > and using the kernel-internal types at least makes it consistent
> > with the rest of the kernel headers.
> > 
> > If we can rely on compiling with a modern compiler (any version of
> > clang, or gcc-4.5+), it predefines a __UINT64_TYPE__ macro that
> > could be used for custom typedef:
> > 
> > #ifdef __UINT64_TYPE__
> > typedef __UINT64_TYPE__		fuse_u64;
> > typedef __INT64_TYPE__		fuse_s64;
> > typedef __UINT32_TYPE__		fuse_u32;
> > typedef __INT32_TYPE__		fuse_s32;
> > ...
> > #else
> > #include <stdint.h>
> > typedef uint64_t		fuse_u64;
> > typedef int64_t			fuse_s64;
> > typedef uint32_t		fuse_u32;
> > typedef int32_t			fuse_s32;
> > ...
> > #endif
> 
> I personally like this version.

Ack, I'll use this. Although I am not sure why uint64_t and __UINT64_TYPE__
should be guaranteed to be identical.

> > The #else side could perhaps be left out here.
> 
> Maybe we should keep it for safety? Less for kernel, but more for
> userspace- we don't know in which environments/libs the header is used.

Ack.

> >> Maybe you could explain your issue more detailed? I.e. how are you using
> >> this include exactly?
> > 
> > I'm interested specifically in two aspects:
> > 
> > - being able to build-test all kernel headers for continuous
> >   integration testing, without having to have access to libc
> >   headers for each target architecture when cross compiling.
> > 
> > - layering kernel headers such that kernel headers never depend
> >   on libc headers and (in a later stage) any kernel header
> >   can be included without clashing with libc definitions. 
> 
> Thank you, I think it would good to add these details to the commit message.

Ack.


Thomas