From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-yw0-f199.google.com (mail-yw0-f199.google.com [209.85.161.199]) by kanga.kvack.org (Postfix) with ESMTP id 490D56B025F for ; Tue, 15 Aug 2017 12:24:26 -0400 (EDT) Received: by mail-yw0-f199.google.com with SMTP id c206so19252412ywb.12 for ; Tue, 15 Aug 2017 09:24:26 -0700 (PDT) Received: from mail-yw0-x233.google.com (mail-yw0-x233.google.com. [2607:f8b0:4002:c05::233]) by mx.google.com with ESMTPS id g141si2656447ywe.87.2017.08.15.09.24.24 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 15 Aug 2017 09:24:25 -0700 (PDT) Received: by mail-yw0-x233.google.com with SMTP id s143so7677506ywg.1 for ; Tue, 15 Aug 2017 09:24:24 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20170815122701.GF27505@quack2.suse.cz> References: <150277752553.23945.13932394738552748440.stgit@dwillia2-desk3.amr.corp.intel.com> <150277753660.23945.11500026891611444016.stgit@dwillia2-desk3.amr.corp.intel.com> <20170815122701.GF27505@quack2.suse.cz> From: Dan Williams Date: Tue, 15 Aug 2017 09:24:24 -0700 Message-ID: Subject: Re: [PATCH v4 2/3] mm: introduce MAP_VALIDATE a mechanism for adding new mmap flags Content-Type: text/plain; charset="UTF-8" Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: "Darrick J. Wong" , Arnd Bergmann , "linux-nvdimm@lists.01.org" , Linux API , "linux-kernel@vger.kernel.org" , linux-xfs@vger.kernel.org, Linux MM , Andy Lutomirski , linux-fsdevel , Andrew Morton , Christoph Hellwig On Tue, Aug 15, 2017 at 5:27 AM, Jan Kara 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... Changing the prototype is relatively easy with Coccinelle, but we still need the code in each ->mmap() implementation to validate a local list of supported flags. How about adding a 'supported mmap flags' field to 'struct file_operations' so that the validation code can be made generic? I'll go with that since it's a bit less surprising than a new operation type, and not as messy as teaching every mmap implementation in the kernel to validate flags that they will likely never care about. > Also for MAP_SYNC I want the flag to be copied in VMA anyway so for that I > don't need additional flags argument anyway. And I wonder how you want to > make things work without VMA flag in case of MAP_DIRECT as well - VMAs can > be split, partially unmapped etc. and so without VMA flag you are going to > have hard time to detect whether there's any mapping left which blocks > block mapping changes. Outside of requiring a 64-bit arch, we're out of vm_flags. Also, the core mm does not really care about MAP_DIRECT or MAP_SYNC so that's why I added a new ->fs_flags field since these are more filesystem properties than core mm. The problem of tracking MAP_DIRECT over vma splits appears to already be handled. __split_vma does: /* most fields are the same, copy all, and then fixup */ *new = *vma; ... if (new->vm_ops && new->vm_ops->open) new->vm_ops->open(new); In ->open() I'm checking if 'new' has MAP_DIRECT in ->fs_flags and taking a reference against the S_IOMAP_SEALED flag. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org