Re: [PATCH bpf-next] bpf, riscv: Fix stack layout of JITed code on RV32

[Daniel Borkmann]

On 4/30/20 4:14 AM, Xi Wang wrote:
> On Wed, Apr 29, 2020 at 5:51 PM Luke Nelson <lukenels@xxxxxxxxxxxxxxxxx> wrote:
> > This patch fixes issues with stackframe unwinding and alignment in the
> > current stack layout for BPF programs on RV32.
> >
> > In the current layout, RV32 fp points to the JIT scratch registers, rather
> > than to the callee-saved registers. This breaks stackframe unwinding,
> > which expects fp to point just above the saved ra and fp registers.
> >
> > This patch fixes the issue by moving the callee-saved registers to be
> > stored on the top of the stack, pointed to by fp. This satisfies the
> > assumptions of stackframe unwinding.
> >
> > This patch also fixes an issue with the old layout that the stack was
> > not aligned to 16 bytes.
> >
> > Stacktrace from JITed code using the old stack layout:
> >
> >    [   12.196249 ] [<c0402200>] walk_stackframe+0x0/0x96
> >
> > Stacktrace using the new stack layout:
> >
> >    [   13.062888 ] [<c0402200>] walk_stackframe+0x0/0x96
> >    [   13.063028 ] [<c04023c6>] show_stack+0x28/0x32
> >    [   13.063253 ] [<a403e778>] bpf_prog_82b916b2dfa00464+0x80/0x908
> >    [   13.063417 ] [<c09270b2>] bpf_test_run+0x124/0x39a
> >    [   13.063553 ] [<c09276c0>] bpf_prog_test_run_skb+0x234/0x448
> >    [   13.063704 ] [<c048510e>] __do_sys_bpf+0x766/0x13b4
> >    [   13.063840 ] [<c0485d82>] sys_bpf+0xc/0x14
> >    [   13.063961 ] [<c04010f0>] ret_from_syscall+0x0/0x2
> >
> > The new code is also simpler to understand and includes an ASCII diagram
> > of the stack layout.
> >
> > Tested on riscv32 QEMU virt machine.
> >
> > Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>
>
> Thanks for the fix!
>
> Acked-by: Xi Wang <xi.wang@xxxxxxxxx> 

Applied, thanks everyone!