On Tue 10-10-17 07:49:01, Dan Williams wrote: > The mmap(2) 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 support. Define a new MAP_SHARED_VALIDATE flag pattern that > is guaranteed to fail on all legacy mmap implementations. > > It is worth noting that the original proposal was for a standalone > MAP_VALIDATE flag. However, when that could not be supported by all > archs Linus observed: > > I see why you *think* you want a bitmap. You think you want > a bitmap because you want to make MAP_VALIDATE be part of MAP_SYNC > etc, so that people can do > > ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED > | MAP_SYNC, fd, 0); > > and "know" that MAP_SYNC actually takes. > > And I'm saying that whole wish is bogus. You're fundamentally > depending on special semantics, just make it explicit. It's already > not portable, so don't try to make it so. > > Rename that MAP_VALIDATE as MAP_SHARED_VALIDATE, make it have a value > of 0x3, and make people do > > ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED_VALIDATE > | MAP_SYNC, fd, 0); > > and then the kernel side is easier too (none of that random garbage > playing games with looking at the "MAP_VALIDATE bit", but just another > case statement in that map type thing. > > Boom. Done. > > Similar to ->fallocate() we also want the ability to validate the > support for new flags on a per ->mmap() 'struct file_operations' > instance basis. Towards that end arrange for flags to be generically > validated against a mmap_supported_mask exported by 'struct > file_operations'. By default all existing flags are implicitly > supported, but new flags require MAP_SHARED_VALIDATE and > per-instance-opt-in. > > Cc: Jan Kara <jack@xxxxxxx> > Cc: Arnd Bergmann <arnd@xxxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Suggested-by: Christoph Hellwig <hch@xxxxxx> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx> > --- > arch/alpha/include/uapi/asm/mman.h | 1 + > arch/mips/include/uapi/asm/mman.h | 1 + > arch/mips/kernel/vdso.c | 2 + > arch/parisc/include/uapi/asm/mman.h | 1 + > arch/tile/mm/elf.c | 3 +- > arch/xtensa/include/uapi/asm/mman.h | 1 + > include/linux/fs.h | 2 + > include/linux/mm.h | 2 + > include/linux/mman.h | 39 ++++++++++++++++++++++++++ > include/uapi/asm-generic/mman-common.h | 1 + > mm/mmap.c | 21 ++++++++++++-- > tools/include/uapi/asm-generic/mman-common.h | 1 + > 12 files changed, 69 insertions(+), 6 deletions(-) > > diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h > index 3b26cc62dadb..92823f24890b 100644 > --- a/arch/alpha/include/uapi/asm/mman.h > +++ b/arch/alpha/include/uapi/asm/mman.h > @@ -14,6 +14,7 @@ > #define MAP_TYPE 0x0f /* Mask for type of mapping (OSF/1 is _wrong_) */ > #define MAP_FIXED 0x100 /* Interpret addr exactly */ > #define MAP_ANONYMOUS 0x10 /* don't use a file */ > +#define MAP_SHARED_VALIDATE 0x3 /* share + validate extension flags */ Just a nit but I'd put definition of MAP_SHARED_VALIDATE close to the definition of MAP_SHARED and MAP_PRIVATE where it logically belongs (for all archs). > diff --git a/include/linux/mm.h b/include/linux/mm.h > index f8c10d336e42..5c4c98e4adc9 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -2133,7 +2133,7 @@ extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned lo > > extern unsigned long mmap_region(struct file *file, unsigned long addr, > unsigned long len, vm_flags_t vm_flags, unsigned long pgoff, > - struct list_head *uf); > + struct list_head *uf, unsigned long map_flags); > extern unsigned long do_mmap(struct file *file, unsigned long addr, > unsigned long len, unsigned long prot, unsigned long flags, > vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate, I have to say I'm not very keen on passing down both vm_flags and map_flags - vm_flags are almost a subset of map_flags but not quite and the ambiguity which needs to be used for a particular check seems to open a space for errors. Granted you currently only care about MAP_DIRECT in ->mmap_validate and just pass map_flags through mmap_region() so there's no space for confusion but future checks could do something different. But OTOH I don't see a cleaner way of avoiding the need to allocate vma flag for something you need to check down in ->mmap_validate so I guess I'll live with that and if problems really happen, we may have cleaner idea what needs to be done. So overall feel free to add: Reviewed-by: Jan Kara <jack@xxxxxxx> Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
