On Mon, May 11, 2026 at 6:00 PM Lorenzo Stoakes <[email protected]> wrote: > On Mon, May 11, 2026 at 05:40:24PM +0200, Jann Horn wrote: > > On Mon, May 11, 2026 at 5:32 PM Lorenzo Stoakes <[email protected]> wrote: > > > On Mon, May 11, 2026 at 05:19:50PM +0200, Jann Horn wrote: > > > > On Mon, May 11, 2026 at 5:05 PM Lorenzo Stoakes <[email protected]> wrote: > > > > > Hmmm I think it's a bit debateable honestly. The ability to handle > > > > > there being > > > > > gaps is a _new thing_, so there are no semantics to speak of. > > > > > Prevoiusly > > > > > mremap() simply required that you only span across a single VMA. > > > > > > > > FWIW, I think mremap() on a source region with gaps is such a > > > > hazardous operation that nearly no userspace code should be doing it - > > > > gaps are areas in which any mmap() call without a fixed address could > > > > place unrelated mappings (unless stack VMAs are involved, which would > > > > also be a weird scenario), so to use it safely, you have to, among > > > > other things, make sure not to use libc malloc() at a time when that > > > > could place an allocation in the gap (which means you also can't use > > > > printf(), and so on, unless you have swapped out the memory > > > > allocator), and make sure that you have no other threads that could be > > > > doing that, and so on. There are rare circumstances under which it > > > > could be safe, but I think it is almost always better to have a > > > > PROT_NONE anonymous VMA or such as a placeholder. > > > > > > Well, we're holding the mmap write lock so none of that could happen > > > _during_ the operation right? > > > > Not during the operation, but right before the operation. So from the > > userspace perspective, you have to know that there are no concurrent > > threads that could be creating memory mappings at non-fixed addresses, > > and you have to know that no mappings can have been created in the > > memory range between when you checked that it's empty and when you > > make the syscall. > > That's a very good point :) > > But I guess applies to any operations that operate over a range of mappings > anyway (madvise() lets you also do this, though it'll give an error code > _at the end_ _after having done the operations_ if there are gaps). > > So madvise() can have the exact same thing happen right? which is... fun :) > > I actually wonder if we shouldn't just change this to disallow gaps. It'd
That would be nice... > simplify the code and we could even do the check upfront in one pass. It's > doubtful anybody is relying on the gaps behaviour for anything real. (I remember that in the past, there were concerns that MM operations with multiple passes are slow, but I guess mremap() is probably not such a hot operation that that's a concern here.)
