git: 6b49d1194899 - main - kern: allow kern_shm_open2 of an anonymous preconstructed shmfd

The branch main has been updated by kevans:

URL: https://cgit.FreeBSD.org/src/commit/?id=6b49d119489944abbef87b5892c21f65e7a7055e

commit 6b49d119489944abbef87b5892c21f65e7a7055e
Author:     Kyle Evans <kevans@FreeBSD.org>
AuthorDate: 2025-07-14 23:28:35 +0000
Commit:     Kyle Evans <kevans@FreeBSD.org>
CommitDate: 2025-07-26 21:31:40 +0000

    kern: allow kern_shm_open2 of an anonymous preconstructed shmfd
    
    The motivation here is for future changes to the coredump code to be
    able to build up a coredump into a shmfd instead of a vnode, which then
    gets tapped out to userland via a character device.  This also opens up
    the possibility that it's useful for the kernel to be able to construct
    a shmfd and pass it out to a process that shouldn't need to write to it.
    
    Reviewed by:    emaste, kib, markj
    Differential Revision:  https://reviews.freebsd.org/D51336
---
 sys/compat/linux/linux_file.c |  2 +-
 sys/kern/uipc_shm.c           | 50 +++++++++++++++++++++++++++++--------------
 sys/sys/syscallsubr.h         |  3 ++-
 3 files changed, 37 insertions(+), 18 deletions(-)

diff --git a/sys/compat/linux/linux_file.c b/sys/compat/linux/linux_file.c
index 86834a7ecea8..a4be5313aa96 100644
--- a/sys/compat/linux/linux_file.c
+++ b/sys/compat/linux/linux_file.c
@@ -1792,7 +1792,7 @@ linux_memfd_create(struct thread *td, struct linux_memfd_create_args *args)
 	if ((flags & MFD_ALLOW_SEALING) != 0)
 		shmflags |= SHM_ALLOW_SEALING;
 	return (kern_shm_open2(td, SHM_ANON, oflags, 0, shmflags, NULL,
-	    memfd_name));
+	    memfd_name, NULL));
 }
 
 int
diff --git a/sys/kern/uipc_shm.c b/sys/kern/uipc_shm.c
index 6f83b875a6b6..85fe48ddd466 100644
--- a/sys/kern/uipc_shm.c
+++ b/sys/kern/uipc_shm.c
@@ -1134,10 +1134,10 @@ shm_doremove(struct shm_mapping *map)
 
 int
 kern_shm_open2(struct thread *td, const char *userpath, int flags, mode_t mode,
-    int shmflags, struct filecaps *fcaps, const char *name __unused)
+    int shmflags, struct filecaps *fcaps, const char *name __unused,
+    struct shmfd *shmfd)
 {
 	struct pwddesc *pdp;
-	struct shmfd *shmfd;
 	struct file *fp;
 	char *path;
 	void *rl_cookie;
@@ -1214,23 +1214,41 @@ kern_shm_open2(struct thread *td, const char *userpath, int flags, mode_t mode,
 	if (error != 0)
 		goto outnofp;
 
-	/* A SHM_ANON path pointer creates an anonymous object. */
+	/*
+	 * A SHM_ANON path pointer creates an anonymous object.  We allow other
+	 * parts of the kernel to pre-populate a shmfd and then materialize an
+	 * fd for it here as a means to pass data back up to userland.  This
+	 * doesn't really make sense for named shm objects, but it makes plenty
+	 * of sense for anonymous objects.
+	 */
 	if (userpath == SHM_ANON) {
-		/* A read-only anonymous object is pointless. */
-		if ((flags & O_ACCMODE) == O_RDONLY) {
-			error = EINVAL;
-			goto out;
-		}
-		shmfd = shm_alloc(td->td_ucred, cmode, largepage);
-		if (shmfd == NULL) {
-			error = ENOMEM;
-			goto out;
+		if (shmfd != NULL) {
+			shm_hold(shmfd);
+		} else {
+			/*
+			 * A read-only anonymous object is pointless, unless it
+			 * was pre-populated by the kernel with the expectation
+			 * that a shmfd would later be created for userland to
+			 * access it through.
+			 */
+			if ((flags & O_ACCMODE) == O_RDONLY) {
+				error = EINVAL;
+				goto out;
+			}
+			shmfd = shm_alloc(td->td_ucred, cmode, largepage);
+			if (shmfd == NULL) {
+				error = ENOMEM;
+				goto out;
+			}
+
+			shmfd->shm_seals = initial_seals;
+			shmfd->shm_flags = shmflags;
 		}
-		shmfd->shm_seals = initial_seals;
-		shmfd->shm_flags = shmflags;
 	} else {
 		fnv = fnv_32_str(path, FNV1_32_INIT);
 		sx_xlock(&shm_dict_lock);
+
+		MPASS(shmfd == NULL);
 		shmfd = shm_lookup(path, fnv);
 		if (shmfd == NULL) {
 			/* Object does not yet exist, create it if requested. */
@@ -2173,7 +2191,7 @@ kern_shm_open(struct thread *td, const char *path, int flags, mode_t mode,
     struct filecaps *caps)
 {
 
-	return (kern_shm_open2(td, path, flags, mode, 0, caps, NULL));
+	return (kern_shm_open2(td, path, flags, mode, 0, caps, NULL, NULL));
 }
 
 /*
@@ -2191,7 +2209,7 @@ sys_shm_open2(struct thread *td, struct shm_open2_args *uap)
 {
 
 	return (kern_shm_open2(td, uap->path, uap->flags, uap->mode,
-	    uap->shmflags, NULL, uap->name));
+	    uap->shmflags, NULL, uap->name, NULL));
 }
 
 int
diff --git a/sys/sys/syscallsubr.h b/sys/sys/syscallsubr.h
index fd183ffbc7a4..8237165b84ce 100644
--- a/sys/sys/syscallsubr.h
+++ b/sys/sys/syscallsubr.h
@@ -60,6 +60,7 @@ struct rusage;
 struct sched_param;
 struct sembuf;
 union semun;
+struct shmfd;
 struct sockaddr;
 struct spacectl_range;
 struct stat;
@@ -337,7 +338,7 @@ int	kern_shm_open(struct thread *td, const char *userpath, int flags,
 	    mode_t mode, struct filecaps *fcaps);
 int	kern_shm_open2(struct thread *td, const char *path, int flags,
 	    mode_t mode, int shmflags, struct filecaps *fcaps,
-	    const char *name);
+	    const char *name, struct shmfd *shmfd);
 int	kern_shmat(struct thread *td, int shmid, const void *shmaddr,
 	    int shmflg);
 int	kern_shmctl(struct thread *td, int shmid, int cmd, void *buf,