[PATCH 1/4] bpf: account for freed JIT allocations in arch code

[Ard Biesheuvel]

On Mon, 19 Nov 2018 at 02:37, Daniel Borkmann <daniel at iogearbox.net> wrote:
>
> On 11/17/2018 07:57 PM, Ard Biesheuvel wrote:
> > Commit ede95a63b5e84 ("bpf: add bpf_jit_limit knob to restrict unpriv
> > allocations") added a call to bpf_jit_uncharge_modmem() to the routine
> > bpf_jit_binary_free() which is called from the __weak bpf_jit_free().
> > This function is overridden by arches, some of which do not call
> > bpf_jit_binary_free() to release the memory, and so the released
> > memory is not accounted for, potentially leading to spurious allocation
> > failures.
> >
> > So replace the direct calls to module_memfree() in the arch code with
> > calls to bpf_jit_binary_free().
>
> Sorry but this patch is completely buggy, and above description on the
> accounting incorrect as well. Looks like this patch was not tested at all.
>

My apologies. I went off into the weeds a bit looking at different
versions for 32-bit and 64-bit on different architectures. So indeed,
this patch should be dropped.

> The below cBPF JITs that use module_memfree() which you replace with
> bpf_jit_binary_free() are using module_alloc() internally to get the JIT
> image buffer ...
>

Indeed. So would you prefer for arm64 to override bpf_jit_free() in
its entirety, and not call bpf_jit_binary_free() but simply call
bpf_jit_uncharge_modmem() and vfree() directly? It's either that, or
we'd have to untangle this a bit, to avoid having one __weak function
on top of the other just so other arches can replace the
module_memfree() call in bpf_jit_binary_free() with vfree() (which
amount to the same thing on arm64 anyway)