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openat2: LINUX_VERSION_CODE is defined by linux-libc-dev, not libc

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With new kernel headers and old libc, `SYS_openat2` can still end up
undefined, which triggers the fallback build-time code, that doesn't build:

```
openat2.cc: In function 'int openat2(int, const char*, open_how*, size_t)':
openat2.cc:35:2: error: 'errno' was not declared in this scope
   35 |  errno = ENOSYS;
      |  ^~~~~
openat2.cc:24:1: note: 'errno' is defined in header '<cerrno>'; did you forget to '#include <cerrno>'?
   23 | #include <unistd.h>
  +++ |+#include <cerrno>
   24 |
openat2.cc:35:10: error: 'ENOSYS' was not declared in this scope
   35 |  errno = ENOSYS;
      |          ^~~~~~
openat2.cc:29:19: error: unused parameter 'dirfd' [-Werror=unused-parameter]
   29 | openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size)
      |         ~~~~~~~~~~^~~~~
openat2.cc:29:44: error: unused parameter 'pathname' [-Werror=unused-parameter]
   29 | openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size)
      |                          ~~~~~~~~~~~~~~~~~~^~~~~~~~
openat2.cc:29:77: error: unused parameter 'how' [-Werror=unused-parameter]
   29 | t dirfd, const char *const pathname, struct open_how *const how, size_t const size)
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~^~~

openat2.cc:29:95: error: unused parameter 'size' [-Werror=unused-parameter]
   29 | st char *const pathname, struct open_how *const how, size_t const size)
      |                                                      ~~~~~~~~~~~~~^~~~
```

Skip the kernel version check and test for the definition of `SYS_openat2`
directly.  If it's not there, plug in the constant so we can send the
call directly to the kernel, bypassing libc completely.

Fixes: https://github.com/Zygo/bees/issues/318
Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell
2025-07-07 14:27:57 -04:00
parent 5c0480ec59
commit e852e3998a
1 changed files with 1 additions and 10 deletions
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11
lib/openat2.cc
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@@ -4,9 +4,7 @@
// Compatibility for building on old libc for new kernel // Compatibility for building on old libc for new kernel
#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 6, 0) // Every arch that defines this (so far) uses 437, except Alpha, where 437 is
// Every arch that defines this uses 437, except Alpha, where 437 is
// mq_getsetattr. // mq_getsetattr.
#ifndef SYS_openat2 #ifndef SYS_openat2
@@ -17,8 +15,6 @@
#endif #endif
#endif #endif
#endif // Linux version >= v5.6
#include <fcntl.h> #include <fcntl.h>
#include <unistd.h> #include <unistd.h>
@@ -29,12 +25,7 @@ __attribute__((weak))
openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size) openat2(int const dirfd, const char *const pathname, struct open_how *const how, size_t const size)
throw() throw()
{ {
#ifdef SYS_openat2
return syscall(SYS_openat2, dirfd, pathname, how, size); return syscall(SYS_openat2, dirfd, pathname, how, size);
#else
errno = ENOSYS;
return -1;
#endif
} }
}; };