Hi Chao, On Wed, Jul 6, 2022 at 9:25 AM Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> wrote: > > Normally, a write to unallocated space of a file or the hole of a sparse > file automatically causes space allocation, for memfd, this equals to > memory allocation. This new seal prevents such automatically allocating, > either this is from a direct write() or a write on the previously > mmap-ed area. The seal does not prevent fallocate() so an explicit > fallocate() can still cause allocating and can be used to reserve > memory. > > This is used to prevent unintentional allocation from userspace on a > stray or careless write and any intentional allocation should use an > explicit fallocate(). One of the main usecases is to avoid memory double > allocation for confidential computing usage where we use two memfds to > back guest memory and at a single point only one memfd is alive and we > want to prevent memory allocation for the other memfd which may have > been mmap-ed previously. More discussion can be found at: > > https://lkml.org/lkml/2022/6/14/1255 > > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> > --- > include/uapi/linux/fcntl.h | 1 + > mm/memfd.c | 3 ++- > mm/shmem.c | 16 ++++++++++++++-- > 3 files changed, 17 insertions(+), 3 deletions(-) > > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h > index 2f86b2ad6d7e..98bdabc8e309 100644 > --- a/include/uapi/linux/fcntl.h > +++ b/include/uapi/linux/fcntl.h > @@ -43,6 +43,7 @@ > #define F_SEAL_GROW 0x0004 /* prevent file from growing */ > #define F_SEAL_WRITE 0x0008 /* prevent writes */ > #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */ > +#define F_SEAL_AUTO_ALLOCATE 0x0020 /* prevent allocation for writes */ I think this should also be added to tools/include/uapi/linux/fcntl.h Cheers, /fuad > /* (1U << 31) is reserved for signed error codes */ > > /* > diff --git a/mm/memfd.c b/mm/memfd.c > index 08f5f8304746..2afd898798e4 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -150,7 +150,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file) > F_SEAL_SHRINK | \ > F_SEAL_GROW | \ > F_SEAL_WRITE | \ > - F_SEAL_FUTURE_WRITE) > + F_SEAL_FUTURE_WRITE | \ > + F_SEAL_AUTO_ALLOCATE) > > static int memfd_add_seals(struct file *file, unsigned int seals) > { > diff --git a/mm/shmem.c b/mm/shmem.c > index a6f565308133..6c8aef15a17d 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -2051,6 +2051,8 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf) > struct vm_area_struct *vma = vmf->vma; > struct inode *inode = file_inode(vma->vm_file); > gfp_t gfp = mapping_gfp_mask(inode->i_mapping); > + struct shmem_inode_info *info = SHMEM_I(inode); > + enum sgp_type sgp; > int err; > vm_fault_t ret = VM_FAULT_LOCKED; > > @@ -2113,7 +2115,12 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf) > spin_unlock(&inode->i_lock); > } > > - err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, SGP_CACHE, > + if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE)) > + sgp = SGP_NOALLOC; > + else > + sgp = SGP_CACHE; > + > + err = shmem_getpage_gfp(inode, vmf->pgoff, &vmf->page, sgp, > gfp, vma, vmf, &ret); > if (err) > return vmf_error(err); > @@ -2459,6 +2466,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping, > struct inode *inode = mapping->host; > struct shmem_inode_info *info = SHMEM_I(inode); > pgoff_t index = pos >> PAGE_SHIFT; > + enum sgp_type sgp; > int ret = 0; > > /* i_rwsem is held by caller */ > @@ -2470,7 +2478,11 @@ shmem_write_begin(struct file *file, struct address_space *mapping, > return -EPERM; > } > > - ret = shmem_getpage(inode, index, pagep, SGP_WRITE); > + if (unlikely(info->seals & F_SEAL_AUTO_ALLOCATE)) > + sgp = SGP_NOALLOC; > + else > + sgp = SGP_WRITE; > + ret = shmem_getpage(inode, index, pagep, sgp); > > if (ret) > return ret; > -- > 2.25.1 >
