On Tue, Sep 15, 2015 at 03:23:58AM +0300, Kirill A. Shutemov wrote: > On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote: > > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so > > userspace wouldn't actually ever see it be non-zero. While I could > > have kept hiding it, the man pages seem to indicate that > > MAP_UNINITIALIZED should be visible: > > > > mmap(2) > > MAP_UNINITIALIZED (since Linux 2.6.33) > > Don't clear anonymous pages. This flag is intended to improve > > performance on embedded devices. This flag is honored only if the > > kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED > > option. Because of the security implications, that option is > > normally enabled only on embedded devices (i.e., devices where one > > has complete control of the contents of user memory). > > > > and since the only time it shows up in my /usr/include is in this > > header I believe this should have been visible to userspace (as > > non-zero, which wouldn't do anything when or'd into the flags) all > > along. > > Are you sure about "wouldn't do anything"? > Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any > architecture has order-1 huge pages, but still looks like we have conflict > here. > > I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which > potentially can handle multiple users. Or non-trivial user space in > general. The flag should always exist. If it was defined to conflict with something else, that's a serious ABI problem. But the flag should always exist, even if the kernel ends up ignoring it. > Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's > possible to have single ABI for MMU and MMU-less systems anyway. And we > can avoid conflict with MAP_HUGE_SHIFT this way. No; even if you have an MMU (which is useful for things like fork()), a system without user separation (for instance, without CONFIG_MULTIUSER) can reasonably use MAP_UNINITIALIZED. > P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig > mailing list on why it was allowed. That's what the config option *and* explicit flag are for; there are more than enough warnings about the implications. - Josh Triplett -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
