On Sun, Jun 25, 2023 at 09:59:34AM -0700, Andy Lutomirski wrote: > > > On Sun, Jun 25, 2023, at 9:14 AM, Mike Rapoport wrote: > > On Mon, Jun 19, 2023 at 10:09:02AM -0700, Andy Lutomirski wrote: > >> > >> On Sun, Jun 18, 2023, at 1:00 AM, Mike Rapoport wrote: > >> > On Sat, Jun 17, 2023 at 01:38:29PM -0700, Andy Lutomirski wrote: > >> >> On Fri, Jun 16, 2023, at 1:50 AM, Mike Rapoport wrote: > >> >> > From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx> > >> >> > > >> >> > module_alloc() is used everywhere as a mean to allocate memory for code. > >> >> > > >> >> > Beside being semantically wrong, this unnecessarily ties all subsystems > >> >> > that need to allocate code, such as ftrace, kprobes and BPF to modules > >> >> > and puts the burden of code allocation to the modules code. > >> >> > > >> >> > Several architectures override module_alloc() because of various > >> >> > constraints where the executable memory can be located and this causes > >> >> > additional obstacles for improvements of code allocation. > >> >> > > >> >> > Start splitting code allocation from modules by introducing > >> >> > execmem_text_alloc(), execmem_free(), jit_text_alloc(), jit_free() APIs. > >> >> > > >> >> > Initially, execmem_text_alloc() and jit_text_alloc() are wrappers for > >> >> > module_alloc() and execmem_free() and jit_free() are replacements of > >> >> > module_memfree() to allow updating all call sites to use the new APIs. > >> >> > > >> >> > The intention semantics for new allocation APIs: > >> >> > > >> >> > * execmem_text_alloc() should be used to allocate memory that must reside > >> >> > close to the kernel image, like loadable kernel modules and generated > >> >> > code that is restricted by relative addressing. > >> >> > > >> >> > * jit_text_alloc() should be used to allocate memory for generated code > >> >> > when there are no restrictions for the code placement. For > >> >> > architectures that require that any code is within certain distance > >> >> > from the kernel image, jit_text_alloc() will be essentially aliased to > >> >> > execmem_text_alloc(). > >> >> > > >> >> > >> >> Is there anything in this series to help users do the appropriate > >> >> synchronization when the actually populate the allocated memory with > >> >> code? See here, for example: > >> > > >> > This series only factors out the executable allocations from modules and > >> > puts them in a central place. > >> > Anything else would go on top after this lands. > >> > >> Hmm. > >> > >> On the one hand, there's nothing wrong with factoring out common code. On > >> the other hand, this is probably the right time to at least start > >> thinking about synchronization, at least to the extent that it might make > >> us want to change this API. (I'm not at all saying that this series > >> should require changes -- I'm just saying that this is a good time to > >> think about how this should work.) > >> > >> The current APIs, *and* the proposed jit_text_alloc() API, don't actually > >> look like the one think in the Linux ecosystem that actually > >> intelligently and efficiently maps new text into an address space: > >> mmap(). > >> > >> On x86, you can mmap() an existing file full of executable code PROT_EXEC > >> and jump to it with minimal synchronization (just the standard implicit > >> ordering in the kernel that populates the pages before setting up the > >> PTEs and whatever user synchronization is needed to avoid jumping into > >> the mapping before mmap() finishes). It works across CPUs, and the only > >> possible way userspace can screw it up (for a read-only mapping of > >> read-only text, anyway) is to jump to the mapping too early, in which > >> case userspace gets a page fault. Incoherence is impossible, and no one > >> needs to "serialize" (in the SDM sense). > >> > >> I think the same sequence (from userspace's perspective) works on other > >> architectures, too, although I think more cache management is needed on > >> the kernel's end. As far as I know, no Linux SMP architecture needs an > >> IPI to map executable text into usermode, but I could easily be wrong. > >> (IIRC RISC-V has very developer-unfriendly icache management, but I don't > >> remember the details.) > >> > >> Of course, using ptrace or any other FOLL_FORCE to modify text on x86 is > >> rather fraught, and I bet many things do it wrong when userspace is > >> multithreaded. But not in production because it's mostly not used in > >> production.) > >> > >> But jit_text_alloc() can't do this, because the order of operations > >> doesn't match. With jit_text_alloc(), the executable mapping shows up > >> before the text is populated, so there is no atomic change from not-there > >> to populated-and-executable. Which means that there is an opportunity > >> for CPUs, speculatively or otherwise, to start filling various caches > >> with intermediate states of the text, which means that various > >> architectures (even x86!) may need serialization. > >> > >> For eBPF- and module- like use cases, where JITting/code gen is quite > >> coarse-grained, perhaps something vaguely like: > >> > >> jit_text_alloc() -> returns a handle and an executable virtual address, > >> but does *not* map it there > >> jit_text_write() -> write to that handle > >> jit_text_map() -> map it and synchronize if needed (no sync needed on > >> x86, I think) > >> > >> could be more efficient and/or safer. > >> > >> (Modules could use this too. Getting alternatives right might take some > >> fiddling, because off the top of my head, this doesn't match how it works > >> now.) > >> > >> To make alternatives easier, this could work, maybe (haven't fully > >> thought it through): > >> > >> jit_text_alloc() > >> jit_text_map_rw_inplace() -> map at the target address, but RW, !X > >> > >> write the text and apply alternatives > >> > >> jit_text_finalize() -> change from RW to RX *and synchronize* > >> > >> jit_text_finalize() would either need to wait for RCU (possibly extra > >> heavy weight RCU to get "serialization") or send an IPI. > > > > This essentially how modules work now. The memory is allocated RW, written > > and updated with alternatives and then made ROX in the end with set_memory > > APIs. > > > > The issue with not having the memory mapped X when it's written is that we > > cannot use large pages to map it. One of the goals is to have executable > > memory mapped with large pages and make code allocator able to divide that > > page among several callers. > > > > So the idea was that jit_text_alloc() will have a cache of large pages > > mapped ROX, will allocate memory from those caches and there will be > > jit_update() that uses text poking for writing to that memory. > > > > Upon allocation of a large page to increase the cache, that large page will > > be "invalidated" by filling it with breakpoint instructions (e.g int3 on > > x86) > > Is this actually valid? In between int3 and real code, there’s a > potential torn read of real code mixed up with 0xcc. You mean while doing text poking? > > To improve the performance of this process, we can write to !X copy and > > then text_poke it to the actual address in one go. This will require some > > changes to get the alternatives right. > > > > -- > > Sincerely yours, > > Mike. -- Sincerely yours, Mike.
