posixshm: implement posix_fallocate(2)

Linux expects to be able to use posix_fallocate(2) on a memfd. Other places
would use this with shm_open(2) to act as a smarter ftruncate(2).

Test has been added to go along with this.

Reviewed by:	kib (earlier version)
Differential Revision:	https://reviews.freebsd.org/D23042

1 files changed, 28 insertions, 0 deletions

diff --git a/sys/kern/uipc_shm.c b/sys/kern/uipc_shm.c
index 435da35a5b28..861e26c85834 100644
--- a/sys/kern/uipc_shm.c
+++ b/sys/kern/uipc_shm.c
@@ -138,6 +138,7 @@ static fo_fill_kinfo_t	shm_fill_kinfo;
 static fo_mmap_t	shm_mmap;
 static fo_get_seals_t	shm_get_seals;
 static fo_add_seals_t	shm_add_seals;
+static fo_fallocate_t	shm_fallocate;
 
 /* File descriptor operations. */
 struct fileops shm_ops = {
@@ -157,6 +158,7 @@ struct fileops shm_ops = {
 	.fo_mmap = shm_mmap,
 	.fo_get_seals = shm_get_seals,
 	.fo_add_seals = shm_add_seals,
+	.fo_fallocate = shm_fallocate,
 	.fo_flags = DFLAG_PASSABLE | DFLAG_SEEKABLE
 };
 
@@ -1438,6 +1440,32 @@ shm_get_seals(struct file *fp, int *seals)
 }
 
 static int
+shm_fallocate(struct file *fp, off_t offset, off_t len, struct thread *td)
+{
+	void *rl_cookie;
+	struct shmfd *shmfd;
+	size_t size;
+	int error;
+
+	/* This assumes that the caller already checked for overflow. */
+	error = 0;
+	shmfd = fp->f_data;
+	size = offset + len;
+	rl_cookie = rangelock_wlock(&shmfd->shm_rl, 0, OFF_MAX,
+	    &shmfd->shm_mtx);
+	if (size > shmfd->shm_size) {
+		VM_OBJECT_WLOCK(shmfd->shm_object);
+		error = shm_dotruncate_locked(shmfd, size, rl_cookie);
+		VM_OBJECT_WUNLOCK(shmfd->shm_object);
+	}
+	rangelock_unlock(&shmfd->shm_rl, rl_cookie, &shmfd->shm_mtx);
+	/* Translate to posix_fallocate(2) return value as needed. */
+	if (error == ENOMEM)
+		error = ENOSPC;
+	return (error);
+}
+
+static int
 sysctl_posix_shm_list(SYSCTL_HANDLER_ARGS)
 {
 	struct shm_mapping *shmm;