Re: [PATCH v3 1/2] tools/nolibc: fcntl: Add fallocate()

Willy Tarreau
2026-05-02T03:00:15Z

On Fri, May 01, 2026 at 09:18:31AM +0100, David Laight wrote:
> On Fri, 1 May 2026 01:41:24 +0900
> Daniel Palmer <daniel@thingy.jp> wrote:
>
> > Add fallocate().
> >
> > Some special care is needed to put the offset and size
> > into the syscall parameters for 32bit machines, x32,
> > and mipsn32.
> >
> > For x32 we can just check if the kernel long size is the
> > same as off_t and use the same path as x86_64.
> >
> > For mipsn32 we override the generic version and provide
> > one that does the right thing.
> >
> > Signed-off-by: Daniel Palmer <daniel@thingy.jp>
> > ---
> > tools/include/nolibc/arch-mips.h | 11 +++++++++++
> > tools/include/nolibc/fcntl.h | 33 ++++++++++++++++++++++++++++++++
> > tools/include/nolibc/sys.h | 8 ++++++++
> > 3 files changed, 52 insertions(+)
> >
> > diff --git a/tools/include/nolibc/arch-mips.h b/tools/include/nolibc/arch-mips.h
> > index 1400653c76c1..e4e42f2bcaf4 100644
> > --- a/tools/include/nolibc/arch-mips.h
> > +++ b/tools/include/nolibc/arch-mips.h
> > @@ -6,6 +6,7 @@
> >
> > #ifndef _NOLIBC_ARCH_MIPS_H
> > #define _NOLIBC_ARCH_MIPS_H
> > +#include <linux/unistd.h>
> >
> > #include "compiler.h"
> > #include "crt.h"
> > @@ -256,6 +257,16 @@
> > _arg4 ? -_num : _num; \
> > })
> >
> > +/* The generic version of this will split offset and size for _ABIN32,
> > + * override it and do the right thing here.
> > + */
> > +static __attribute__((unused))
> > +int _sys_fallocate(int fd, int mode, off_t offset, off_t size)
> > +{
> > + return __nolibc_syscall4(__NR_fallocate, fd, mode, offset, size);
> > +}
> > +#define _sys_fallocate _sys_fallocate
> > +
> > #endif /* _ABIO32 */
> >
> > #ifndef NOLIBC_NO_RUNTIME
> > diff --git a/tools/include/nolibc/fcntl.h b/tools/include/nolibc/fcntl.h
> > index 014910a8e928..dbc99188a49e 100644
> > --- a/tools/include/nolibc/fcntl.h
> > +++ b/tools/include/nolibc/fcntl.h
> > @@ -14,6 +14,9 @@
> > #include "types.h"
> > #include "sys.h"
> >
> > +/* For fallocate() modes */
> > +#include <linux/falloc.h>
> > +
> > /*
> > * int openat(int dirfd, const char *path, int flags[, mode_t mode]);
> > */
> > @@ -80,4 +83,34 @@ int creat(const char *path, mode_t mode)
> > return open(path, O_CREAT | O_WRONLY | O_TRUNC, mode);
> > }
> >
> > +/*
> > + * int fallocate(int fd, int mode, off_t offset, off_t size);
> > + */
> > +
> > +#if !defined(_sys_fallocate)
> > +static __attribute__((unused))
> > +int _sys_fallocate(int fd, int mode, off_t offset, off_t size)
> > +{
> > + /*
> > + * For 32 bit machines __kernel_long_t will be 4, off_t will be 8
> > + * and we need to split offset and size, for 64 machines we can use
> > + * the values as-is.
> > + */
> > + const bool offsetsz_two_args = sizeof(__kernel_long_t) != sizeof(off_t);
>
> I don't think you care about the size of off_t.
> Were it to be 4 the code would be badly wrong.
>
> > +
> > + if (offsetsz_two_args)
> > + return __nolibc_syscall6(__NR_fallocate, fd, mode,
> > + __NOLIBC_LLARGPART(offset, 0), __NOLIBC_LLARGPART(offset, 1),
> > + __NOLIBC_LLARGPART(size, 0), __NOLIBC_LLARGPART(size, 1));
> > + else
> > + return __nolibc_syscall4(__NR_fallocate, fd, mode, offset, size);
> > +}
>
> The above might be more readable as:
> if (sizeof(__kernel_long_t) == 8)
> /* 64 bit, values fit in single arguments */
> return __nolibc_syscall4(__NR_fallocate, fd, mode, offset, size);
>
> /* 32 bit, values need splitting, order depends on endianness */
> /* This test for endianness doesn't rely on any pre-processor defines */
> if (({union {int x; char c;} u; u.x = 1; u.c;}))
> /* Little endian */
> return __nolibc_syscall6(__NR_fallocate, fd, mode,
> offset, offset >> 32, size, size >> 32);
> /* Big endian */
> return __nolibc_syscall6(__NR_fallocate, fd, mode,
> offset >> 32, offset, size >> 32, size);

Honestly David, I find Daniel's version way more readable :-) Precisely
because the repeated variations are abstracted with this more readable
macro. If it was used only once I could possibly agree. Even the
endianness test is hard to read, better rely on __BYTE_ORDER__ for
this.

Cheers,
Willy