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openat2: supply the missing definitions for building with old headers and new kernel

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Apparently Ubuntu 20 has upgraded to kernel 5.15, but still builds things
with 5.4 headers.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2025-01-19 21:09:16 -05:00
parent 874832dc58
commit ad11db2ee1
2 changed files with 63 additions and 3 deletions
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39
include/crucible/openat2.h
View File

@ -1,11 +1,44 @@
#ifndef CRUCIBLE_OPENAT2_H
#define CRUCIBLE_OPENAT2_H
#include <cstdlib>
// Compatibility for building on old libc for new kernel
#include <linux/version.h>
#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
#include <linux/openat2.h>
#include <fcntl.h>
#include <sys/syscall.h>
#include <unistd.h>
#else
#ifndef RESOLVE_NO_XDEV
#define RESOLVE_NO_XDEV 1
// RESOLVE_NO_XDEV was there from the beginning of openat2,
// so if that's missing, so is open_how
struct open_how {
__u64 flags;
__u64 mode;
__u64 resolve;
};
#endif
#ifndef RESOLVE_NO_MAGICLINKS
#define RESOLVE_NO_MAGICLINKS 2
#endif
#ifndef RESOLVE_NO_SYMLINKS
#define RESOLVE_NO_SYMLINKS 4
#endif
#ifndef RESOLVE_BENEATH
#define RESOLVE_BENEATH 8
#endif
#ifndef RESOLVE_IN_ROOT
#define RESOLVE_IN_ROOT 16
#endif
#endif // Linux version >= v5.6
extern "C" {

27
lib/openat2.cc
View File

@ -1,5 +1,27 @@
#include "crucible/openat2.h"
#include <sys/syscall.h>
// Compatibility for building on old libc for new kernel
#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 6, 0)
// Every arch that defines this uses 437, except Alpha, where 437 is
// mq_getsetattr.
#ifndef SYS_openat2
#ifdef __alpha__
#define SYS_openat2 547
#else
#define SYS_openat2 437
#endif
#endif
#endif // Linux version >= v5.6
#include <fcntl.h>
#include <unistd.h>
extern "C" {
int
@ -7,7 +29,12 @@ __attribute__((weak))
openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size)
throw()
{
#ifdef SYS_openat2
return syscall(SYS_openat2, dirfd, pathname, how, size);
#else
errno = ENOSYS;
return -1;
#endif
}
};