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os: use posix_fallocate in creating sharable buffers

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If posix_fallocate is available, use it instead of ftruncate. Unlike
ftruncate, when posix_fallocate succeeds, it guarantees that you cannot
run out of disk space, when later writing to the mmap()'ed file.

With posix_fallocate, if os_create_anonymous_file() succeeds, the
program cannot get a SIGBUS later from accessing this file via mmap. If
there is insufficient disk space, the function fails and errno is set to
ENOSPC.

This is useful on systems, that limit the available buffer space by
having XDG_RUNTIME_DIR on a small tmpfs.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
This commit is contained in:
Pekka Paalanen 2013-11-29 17:48:51 +02:00 committed by Kristian Høgsberg
parent 8a81b83900
commit 5b4ddbc11e
2 changed files with 19 additions and 2 deletions
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2
configure.ac
View file

@ -55,7 +55,7 @@ AC_CHECK_DECL(CLOCK_MONOTONIC,[],
[[#include <time.h>]])
AC_CHECK_HEADERS([execinfo.h])
AC_CHECK_FUNCS([mkostemp strchrnul initgroups])
AC_CHECK_FUNCS([mkostemp strchrnul initgroups posix_fallocate])
COMPOSITOR_MODULES="wayland-server >= 1.3.90 pixman-1"

19
shared/os-compatibility.c
View file

@ -132,6 +132,12 @@ create_tmpfile_cloexec(char *tmpname)
* The file is suitable for buffer sharing between processes by
* transmitting the file descriptor over Unix sockets using the
* SCM_RIGHTS methods.
*
* If the C library implements posix_fallocate(), it is used to
* guarantee that disk space is available for the file at the
* given size. If disk space is insufficent, errno is set to ENOSPC.
* If posix_fallocate() is not supported, program may receive
* SIGBUS on accessing mmap()'ed file contents instead.
*/
int
os_create_anonymous_file(off_t size)
@ -140,6 +146,7 @@ os_create_anonymous_file(off_t size)
const char *path;
char *name;
int fd;
int ret;
path = getenv("XDG_RUNTIME_DIR");
if (!path) {
@ -161,10 +168,20 @@ os_create_anonymous_file(off_t size)
if (fd < 0)
return -1;
if (ftruncate(fd, size) < 0) {
#ifdef HAVE_POSIX_FALLOCATE
ret = posix_fallocate(fd, 0, size);
if (ret != 0) {
close(fd);
errno = ret;
return -1;
}
#else
ret = ftruncate(fd, size);
if (ret < 0) {
close(fd);
return -1;
}
#endif
return fd;
}