On Thu, Oct 19, 2017 at 5:58 AM, Jan Kara <jack@xxxxxxx> wrote: > > From: Dan Williams <dan.j.williams@xxxxxxxxx> > > The mmap(2) syscall suffers from the ABI anti-pattern of not validating > unknown flags. However, proposals like MAP_SYNC 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_flags 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> > Signed-off-by: Jan Kara <jack@xxxxxxx> > --- > arch/alpha/include/uapi/asm/mman.h | 1 + > arch/mips/include/uapi/asm/mman.h | 1 + > arch/parisc/include/uapi/asm/mman.h | 1 + > arch/xtensa/include/uapi/asm/mman.h | 1 + > include/linux/fs.h | 1 + > 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 + > 9 files changed, 67 insertions(+) > > 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 */ Looks good, only comment is your original feedback to move this definition next to MAP_SHARED a few lines up in the all places where we define it. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
