This useful behaviour is implemented for most filesystems, and wants to be implemented for every filesystem, quoth ref: There is general agreement that we should standardize all file systems to prevent modifications even for files that were opened at the time the immutable flag is set. Eventually, a change to enforce this at the VFS layer should be landing in mainline. References: commit 02b016ca7f99 ("ext4: enforce the immutable flag on open files") Signed-off-by: Ahelenia Ziemiańska --- v1: https://lore.kernel.org/linux-fsdevel/znhu3eyffewvvhleewehuvod2wrf4tz6vxrouoakiarjtxt5uy@tarta.nabijaczleweli.xyz/t/#u shmem_page_mkwrite()'s return 0; falls straight into do_page_mkwrite()'s if (unlikely(!(ret & VM_FAULT_LOCKED))) { folio_lock(folio); Given the unlikely, is it better to folio_lock(folio); return VM_FAULT_LOCKED; instead? /ext4# uname -a Linux tarta 6.18.0-10912-g416f99c3b16f-dirty #1 SMP PREEMPT_DYNAMIC Sat Dec 6 12:14:41 CET 2025 x86_64 GNU/Linux /ext4# while sleep 1; do echo $$; done > file & [1] 262 /ext4# chattr +i file /ext4# sh: line 25: echo: write error: Operation not permitted sh: line 25: echo: write error: Operation not permitted sh: line 25: echo: write error: Operation not permitted sh: line 25: echo: write error: Operation not permitted fg while sleep 1; do echo $$; done > file ^C /ext4# mount -t tmpfs tmpfs /tmp /ext4# cd /tmp /tmp# while sleep 1; do echo $$; done > file & [1] 284 /tmp# chattr +i file /tmp# sh: line 35: echo: write error: Operation not permitted sh: line 35: echo: write error: Operation not permitted sh: line 35: echo: write error: Operation not permitted $ cat test.c #include #include #include #include #include int main(int, char **argv) { int fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, 0666); ftruncate(fd, 1024 * 1024); char *addr = mmap(NULL, 1024 * 1024, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); addr[0] = 0x69; int attrs = FS_IMMUTABLE_FL; ioctl(3, FS_IOC_SETFLAGS, &attrs); addr[1024 * 1024 - 1] = 0x69; } # strace ./test /tmp/file execve("./test", ["./test", "/tmp/file"], 0x7ffc720bead8 /* 22 vars */) = 0 ... openat(AT_FDCWD, "/tmp/file", O_RDWR|O_CREAT|O_TRUNC, 0666) = 3 ftruncate(3, 1048576) = 0 mmap(NULL, 1048576, PROT_READ|PROT_WRITE, MAP_SHARED, 3, 0) = 0x7f09bbf2a000 ioctl(3, FS_IOC_SETFLAGS, [FS_IMMUTABLE_FL]) = 0 --- SIGBUS {si_signo=SIGBUS, si_code=BUS_ADRERR, si_addr=0x7f09bc029fff} --- +++ killed by SIGBUS +++ Bus error # tr -d \\0 < /tmp/file; echo i mm/shmem.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/mm/shmem.c b/mm/shmem.c index d578d8e765d7..432935f79f35 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1294,6 +1294,14 @@ static int shmem_setattr(struct mnt_idmap *idmap, bool update_mtime = false; bool update_ctime = true; + if (unlikely(IS_IMMUTABLE(inode))) + return -EPERM; + + if (unlikely(IS_APPEND(inode) && + (attr->ia_valid & (ATTR_MODE | ATTR_UID | + ATTR_GID | ATTR_TIMES_SET)))) + return -EPERM; + error = setattr_prepare(idmap, dentry, attr); if (error) return error; @@ -2763,6 +2771,17 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf) return ret; } +static vm_fault_t shmem_page_mkwrite(struct vm_fault *vmf) +{ + struct file *file = vmf->vma->vm_file; + + if (unlikely(IS_IMMUTABLE(file_inode(file)))) + return VM_FAULT_SIGBUS; + + file_update_time(file); + return 0; +} + unsigned long shmem_get_unmapped_area(struct file *file, unsigned long uaddr, unsigned long len, unsigned long pgoff, unsigned long flags) @@ -3475,6 +3494,10 @@ static ssize_t shmem_file_write_iter(struct kiocb *iocb, struct iov_iter *from) ret = generic_write_checks(iocb, from); if (ret <= 0) goto unlock; + if (unlikely(IS_IMMUTABLE(inode))) { + ret = -EPERM; + goto unlock; + } ret = file_remove_privs(file); if (ret) goto unlock; @@ -5286,6 +5309,7 @@ static const struct super_operations shmem_ops = { static const struct vm_operations_struct shmem_vm_ops = { .fault = shmem_fault, .map_pages = filemap_map_pages, + .page_mkwrite = shmem_page_mkwrite, #ifdef CONFIG_NUMA .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, @@ -5295,6 +5319,7 @@ static const struct vm_operations_struct shmem_vm_ops = { static const struct vm_operations_struct shmem_anon_vm_ops = { .fault = shmem_fault, .map_pages = filemap_map_pages, + .page_mkwrite = shmem_page_mkwrite, #ifdef CONFIG_NUMA .set_policy = shmem_set_policy, .get_policy = shmem_get_policy, -- 2.39.5