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

Daniel Palmer
2026-04-30T16:41:39Z

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);
+
+ 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);
+}
+#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
--
2.53.0