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

David Laight
2026-05-01T08:18:36Z

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);

Although you might want to change/factor out the endianness test.

-- David

> +#endif
> +
> +static __attribute__((unused))
> +int fallocate(int fd, int mode, off_t offset, off_t size)
> +{
> + return __sysret(_sys_fallocate(fd, mode, offset, size));
> +}
> +
> #endif /* _NOLIBC_FCNTL_H */
> diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
> index 33f9c970ae57..b7136a3a7d6a 100644
> --- a/tools/include/nolibc/sys.h
> +++ b/tools/include/nolibc/sys.h
> @@ -29,6 +29,14 @@
> #include "stdarg.h"
> #include "types.h"
>
> +/*
> + * Helper for 32bit machines where a 64bit syscall arg needs to be split into
> + * two 32bit parts while making sure the order of the low/high parts are correct
> + * for the endian:
> + * __NOLIBC_LLARGPART(x, 0), __NOLIBC_LLARGPART(x, 1)
> + */
> +#define __NOLIBC_LLARGPART(_arg, _part) \
> + (((union { long long ll; long l[2]; }) { .ll = _arg }).l[_part])
>
> /* Syscall return helper: takes the syscall value in argument and checks for an
> * error in it. This may only be used with signed returns (int or long), but