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Date: Mon, 18 Sep 2017 11:26:34 +0200 From: Jan Kara <jack@...e.cz> To: Dan Williams <dan.j.williams@...el.com> Cc: Jan Kara <jack@...e.cz>, "Darrick J. Wong" <darrick.wong@...cle.com>, Arnd Bergmann <arnd@...db.de>, "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>, Linux API <linux-api@...r.kernel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, linux-xfs@...r.kernel.org, linux-mm <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>, linux-fsdevel <linux-fsdevel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Christoph Hellwig <hch@....de> Subject: Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags On Sat 16-09-17 20:44:14, Dan Williams wrote: > On Tue, Aug 15, 2017 at 5:27 AM, Jan Kara <jack@...e.cz> wrote: > > On Mon 14-08-17 23:12:16, Dan Williams wrote: > >> The mmap syscall suffers from the ABI anti-pattern of not validating > >> unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT need a > >> mechanism to define new behavior that is known to fail on older kernels > >> without the feature. Use the fact that specifying MAP_SHARED and > >> MAP_PRIVATE at the same time is invalid as a cute hack to allow a new > >> set of validated flags to be introduced. > >> > >> This also introduces the ->fmmap() file operation that is ->mmap() plus > >> flags. Each ->fmmap() implementation must fail requests when a locally > >> unsupported flag is specified. > > ... > >> diff --git a/include/linux/fs.h b/include/linux/fs.h > >> index 1104e5df39ef..bbe755d0caee 100644 > >> --- a/include/linux/fs.h > >> +++ b/include/linux/fs.h > >> @@ -1674,6 +1674,7 @@ struct file_operations { > >> long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long); > >> long (*compat_ioctl) (struct file *, unsigned int, unsigned long); > >> int (*mmap) (struct file *, struct vm_area_struct *); > >> + int (*fmmap) (struct file *, struct vm_area_struct *, unsigned long); > >> int (*open) (struct inode *, struct file *); > >> int (*flush) (struct file *, fl_owner_t id); > >> int (*release) (struct inode *, struct file *); > >> @@ -1748,6 +1749,12 @@ static inline int call_mmap(struct file *file, struct vm_area_struct *vma) > >> return file->f_op->mmap(file, vma); > >> } > >> > >> +static inline int call_fmmap(struct file *file, struct vm_area_struct *vma, > >> + unsigned long flags) > >> +{ > >> + return file->f_op->fmmap(file, vma, flags); > >> +} > >> + > > > > Hum, I dislike a new file op for this when the only problem with ->mmap is > > that it misses 'flags' argument. I understand there are lots of ->mmap > > implementations out there and modifying prototype of them all is painful > > but is it so bad? Coccinelle patch for this should be rather easy... > > So it wasn't all that easy, and Linus declined to take it. I think we > should add a new ->mmap_validate() file operation and save the > tree-wide cleanup until later. Well, we don't even strictly need the flags passed to ->mmap callback if we are willing to use VMA flags. I want to use it for MAP_SYNC anyway... So bumping vma->flags to u64 and using a flag is also an option (and frankly I'd personally just go for that). Honza -- Jan Kara <jack@...e.com> SUSE Labs, CR
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