missing_syscall: when adding syscall replacements, use different names

In meson.build we check that functions are available using:
    meson.get_compiler('c').has_function('foo')
which checks the following:
- if __stub_foo or __stub___foo are defined, return false
- if foo is declared (a pointer to the function can be taken), return true
- otherwise check for __builtin_memfd_create

_stub is documented by glibc as
   It defines a symbol '__stub_FUNCTION' for each function
   in the C library which is a stub, meaning it will fail
   every time called, usually setting errno to ENOSYS.

So if __stub is defined, we know we don't want to use the glibc version, but
this doesn't tell us if the name itself is defined or not. If it _is_ defined,
and we define our replacement as an inline static function, we get an error:

In file included from ../src/basic/missing.h:1358:0,
                 from ../src/basic/util.h:47,
                 from ../src/basic/calendarspec.h:29,
                 from ../src/basic/calendarspec.c:34:
../src/basic/missing_syscall.h:65:19: error: static declaration of 'memfd_create' follows non-static declaration
 static inline int memfd_create(const char *name, unsigned int flags) {
                   ^~~~~~~~~~~~
.../usr/include/bits/mman-shared.h:46:5: note: previous declaration of 'memfd_create' was here
 int memfd_create (const char *__name, unsigned int __flags) __THROW;
     ^~~~~~~~~~~~

To avoid this problem, call our inline functions different than glibc,
and use a #define to map the official name to our replacement.

Fixes systemd#8099.

v2:
- use "missing_" as the prefix instead of "_"

src/basic/missing_syscall.h

@@ -26,9 +26,11 @@
 #include <sys/types.h>
 
 #if !HAVE_PIVOT_ROOT
-static inline int pivot_root(const char *new_root, const char *put_old) {
+static inline int missing_pivot_root(const char *new_root, const char *put_old) {
         return syscall(SYS_pivot_root, new_root, put_old);
 }
+
+#define pivot_root missing_pivot_root
 #endif
 
 /* ======================================================================= */
@@ -62,14 +64,16 @@ static inline int pivot_root(const char *new_root, const char *put_old) {
 #    endif
 #  endif
 
-static inline int memfd_create(const char *name, unsigned int flags) {
+static inline int missing_memfd_create(const char *name, unsigned int flags) {
 #  ifdef __NR_memfd_create
         return syscall(__NR_memfd_create, name, flags);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define memfd_create missing_memfd_create
 #endif
 
 /* ======================================================================= */
@@ -109,22 +113,26 @@ static inline int memfd_create(const char *name, unsigned int flags) {
 #    endif
 #  endif
 
-static inline int getrandom(void *buffer, size_t count, unsigned flags) {
+static inline int missing_getrandom(void *buffer, size_t count, unsigned flags) {
 #  ifdef __NR_getrandom
         return syscall(__NR_getrandom, buffer, count, flags);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define getrandom missing_getrandom
 #endif
 
 /* ======================================================================= */
 
 #if !HAVE_GETTID
-static inline pid_t gettid(void) {
+static inline pid_t missing_gettid(void) {
         return (pid_t) syscall(SYS_gettid);
 }
+
+#define gettid missing_gettid
 #endif
 
 /* ======================================================================= */
@@ -152,14 +160,16 @@ struct file_handle {
         unsigned char f_handle[0];
 };
 
-static inline int name_to_handle_at(int fd, const char *name, struct file_handle *handle, int *mnt_id, int flags) {
+static inline int missing_name_to_handle_at(int fd, const char *name, struct file_handle *handle, int *mnt_id, int flags) {
 #  ifdef __NR_name_to_handle_at
         return syscall(__NR_name_to_handle_at, fd, name, handle, mnt_id, flags);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define name_to_handle_at missing_name_to_handle_at
 #endif
 
 /* ======================================================================= */
@@ -177,14 +187,16 @@ static inline int name_to_handle_at(int fd, const char *name, struct file_handle
 #    endif
 #  endif
 
-static inline int setns(int fd, int nstype) {
+static inline int missing_setns(int fd, int nstype) {
 #  ifdef __NR_setns
         return syscall(__NR_setns, fd, nstype);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define setns missing_setns
 #endif
 
 /* ======================================================================= */
@@ -230,57 +242,68 @@ static inline pid_t raw_getpid(void) {
 #    endif
 #  endif
 
-static inline int renameat2(int oldfd, const char *oldname, int newfd, const char *newname, unsigned flags) {
+static inline int missing_renameat2(int oldfd, const char *oldname, int newfd, const char *newname, unsigned flags) {
 #  ifdef __NR_renameat2
         return syscall(__NR_renameat2, oldfd, oldname, newfd, newname, flags);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define renameat2 missing_renameat2
 #endif
 
 /* ======================================================================= */
 
 #if !HAVE_KCMP
-static inline int kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1, unsigned long idx2) {
+static inline int missing_kcmp(pid_t pid1, pid_t pid2, int type, unsigned long idx1, unsigned long idx2) {
 #  ifdef __NR_kcmp
         return syscall(__NR_kcmp, pid1, pid2, type, idx1, idx2);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define kcmp missing_kcmp
 #endif
 
+
 /* ======================================================================= */
 
 #if !HAVE_KEYCTL
-static inline long keyctl(int cmd, unsigned long arg2, unsigned long arg3, unsigned long arg4,unsigned long arg5) {
+static inline long _keyctl(int cmd, unsigned long arg2, unsigned long arg3, unsigned long arg4,unsigned long arg5) {
 #  ifdef __NR_keyctl
         return syscall(__NR_keyctl, cmd, arg2, arg3, arg4, arg5);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
+
+#define keyctl _keyctl
 }
 
-static inline key_serial_t add_key(const char *type, const char *description, const void *payload, size_t plen, key_serial_t ringid) {
+static inline key_serial_t missing_add_key(const char *type, const char *description, const void *payload, size_t plen, key_serial_t ringid) {
 #  ifdef __NR_add_key
         return syscall(__NR_add_key, type, description, payload, plen, ringid);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
+
+#define add_key missing_add_key
 }
 
-static inline key_serial_t request_key(const char *type, const char *description, const char * callout_info, key_serial_t destringid) {
+static inline key_serial_t missing_request_key(const char *type, const char *description, const char * callout_info, key_serial_t destringid) {
 #  ifdef __NR_request_key
         return syscall(__NR_request_key, type, description, callout_info, destringid);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
+
+#define request_key missing_request_key
 }
 #endif
 
@@ -307,17 +330,19 @@ static inline key_serial_t request_key(const char *type, const char *description
 #    endif
 #  endif
 
-static inline ssize_t copy_file_range(int fd_in, loff_t *off_in,
-                                      int fd_out, loff_t *off_out,
-                                      size_t len,
-                                      unsigned int flags) {
+static inline ssize_t missing_copy_file_range(int fd_in, loff_t *off_in,
+                                              int fd_out, loff_t *off_out,
+                                              size_t len,
+                                              unsigned int flags) {
 #  ifdef __NR_copy_file_range
         return syscall(__NR_copy_file_range, fd_in, off_in, fd_out, off_out, len, flags);
 #  else
         errno = ENOSYS;
         return -1;
 #  endif
 }
+
+#define copy_file_range missing_copy_file_range
 #endif
 
 /* ======================================================================= */
@@ -345,7 +370,7 @@ static inline ssize_t copy_file_range(int fd_in, loff_t *off_in,
 
 union bpf_attr;
 
-static inline int bpf(int cmd, union bpf_attr *attr, size_t size) {
+static inline int missing_bpf(int cmd, union bpf_attr *attr, size_t size) {
 #ifdef __NR_bpf
         return (int) syscall(__NR_bpf, cmd, attr, size);
 #else
@@ -354,6 +379,7 @@ static inline int bpf(int cmd, union bpf_attr *attr, size_t size) {
 #endif
 }
 
+#define bpf missing_bpf
 #endif
 
 /* ======================================================================= */