Re: [PATCH 2/4] net/bpf: refactor freeing of executable allocations

[Ard Biesheuvel]

On Sat, 17 Nov 2018 at 23:47, Y Song <ys114321@xxxxxxxxx> wrote:
>
> On Sat, Nov 17, 2018 at 6:58 PM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> >
> > All arch overrides of the __weak bpf_jit_free() amount to the same
> > thing: the allocated memory was never mapped read-only, and so
> > it does not have to be remapped to read-write before being freed.
> >
> > So in preparation of permitting arches to serve allocations for BPF
> > JIT programs from other regions than the module region, refactor
> > the existing bpf_jit_free() implementations to use the shared code
> > where possible, and only specialize the remap and free operations.
> >
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > ---
> >  arch/mips/net/bpf_jit.c           |  7 ++-----
> >  arch/powerpc/net/bpf_jit_comp.c   |  7 ++-----
> >  arch/powerpc/net/bpf_jit_comp64.c |  9 +++------
> >  arch/sparc/net/bpf_jit_comp_32.c  |  7 ++-----
> >  kernel/bpf/core.c                 | 15 +++++----------
> >  5 files changed, 14 insertions(+), 31 deletions(-)
> >
> > diff --git a/arch/mips/net/bpf_jit.c b/arch/mips/net/bpf_jit.c
> > index 1b69897274a1..5696bd7dccc7 100644
> > --- a/arch/mips/net/bpf_jit.c
> > +++ b/arch/mips/net/bpf_jit.c
> > @@ -1261,10 +1261,7 @@ void bpf_jit_compile(struct bpf_prog *fp)
> >         kfree(ctx.offsets);
> >  }
> >
> > -void bpf_jit_free(struct bpf_prog *fp)
> > +void bpf_jit_binary_free(struct bpf_binary_header *hdr)
> >  {
> > -       if (fp->jited)
> > -               bpf_jit_binary_free(bpf_jit_binary_hdr(fp));
> > -
> > -       bpf_prog_unlock_free(fp);
> > +       module_memfree(hdr);
> >  }
> > diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> > index a1ea1ea6b40d..5b5ce4a1b44b 100644
> > --- a/arch/powerpc/net/bpf_jit_comp.c
> > +++ b/arch/powerpc/net/bpf_jit_comp.c
> > @@ -680,10 +680,7 @@ void bpf_jit_compile(struct bpf_prog *fp)
> >         return;
> >  }
> >
> > -void bpf_jit_free(struct bpf_prog *fp)
> > +void bpf_jit_binary_free(struct bpf_binary_header *hdr)
> >  {
> > -       if (fp->jited)
> > -               bpf_jit_binary_free(bpf_jit_binary_hdr(fp));
> > -
> > -       bpf_prog_unlock_free(fp);
> > +       module_memfree(hdr);
> >  }
> > diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> > index 84c8f013a6c6..f64f1294bd62 100644
> > --- a/arch/powerpc/net/bpf_jit_comp64.c
> > +++ b/arch/powerpc/net/bpf_jit_comp64.c
> > @@ -1021,11 +1021,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> >         return fp;
> >  }
> >
> > -/* Overriding bpf_jit_free() as we don't set images read-only. */
> > -void bpf_jit_free(struct bpf_prog *fp)
> > +/* Overriding bpf_jit_binary_free() as we don't set images read-only. */
> > +void bpf_jit_binary_free(struct bpf_binary_header *hdr)
> >  {
> > -       if (fp->jited)
> > -               bpf_jit_binary_free(bpf_jit_binary_hdr(fp));
> > -
> > -       bpf_prog_unlock_free(fp);
> > +       module_memfree(hdr);
> >  }
> > diff --git a/arch/sparc/net/bpf_jit_comp_32.c b/arch/sparc/net/bpf_jit_comp_32.c
> > index 01bda6bc9e7f..589950d152cc 100644
> > --- a/arch/sparc/net/bpf_jit_comp_32.c
> > +++ b/arch/sparc/net/bpf_jit_comp_32.c
> > @@ -756,10 +756,7 @@ cond_branch:                       f_offset = addrs[i + filter[i].jf];
> >         return;
> >  }
> >
> > -void bpf_jit_free(struct bpf_prog *fp)
> > +void bpf_jit_binary_free(struct bpf_binary_header *hdr)
> >  {
> > -       if (fp->jited)
> > -               bpf_jit_binary_free(bpf_jit_binary_hdr(fp));
> > -
> > -       bpf_prog_unlock_free(fp);
> > +       module_memfree(hdr);
> >  }
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > index 1a796e0799ec..29f766dac203 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -646,25 +646,20 @@ bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
> >         return hdr;
> >  }
> >
> > -void bpf_jit_binary_free(struct bpf_binary_header *hdr)
> > +void __weak bpf_jit_binary_free(struct bpf_binary_header *hdr)
> >  {
> > -       u32 pages = hdr->pages;
> > -
> > +       bpf_jit_binary_unlock_ro(hdr);
> >         module_memfree(hdr);
> > -       bpf_jit_uncharge_modmem(pages);
> >  }
> >
> > -/* This symbol is only overridden by archs that have different
> > - * requirements than the usual eBPF JITs, f.e. when they only
> > - * implement cBPF JIT, do not set images read-only, etc.
> > - */
>
> Do you want to move the above comments to
> new weak function bpf_jit_binary_free?
>

Perhaps. But one thing I don't understand, looking at this again, is
why we have these overrides in the first place. module_memfree() just
calls vfree(), which takes down the mapping entirely (along with any
updated permissions), and so remapping it back to r/w right before
that seems rather pointless imo.

Can we get rid of bpf_jit_binary_unlock_ro() entirely, and along with
it, all these overrides for the free() path?