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

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Message-ID: <20260105092847-f05669a4-f8a1-498d-a8b4-dc1c5a0ac1f8@linutronix.de>
Date: Mon, 5 Jan 2026 09:40:50 +0100
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Bernd Schubert <bernd@...ernd.com>
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 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.


Thomas