Re: [PATCH v2 02/12] mm: introduce execmem_text_alloc() and jit_text_alloc()

[Mike Rapoport <rppt@kernel.org>]

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@kernel.org>
> >> >> >
> >> >> > 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.