On Sun, Jun 25, 2023 at 07:14:17PM +0300, 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) Does that work on x86? That is in no way gauranteed for other architectures; on arm64 you need explicit cache maintenance (with I-cache maintenance at the VA to be executed from) followed by context-synchronization-events (e.g. via ISB instructions, or IPIs). Mark.
