Re: [PATCH 0/2] LoongArch: Make bounds-checking instructions useful

[Huacai Chen]

Hi, Xuerui,

I hope V2 can be applied cleanly without the patch series "LoongArch:
Better backtraces", thanks.

Huacai

On Mon, Apr 17, 2023 at 5:50 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
>
> On Mon, 2023-04-17 at 15:54 +0800, WANG Xuerui wrote:
> > On 2023/4/17 14:47, Xi Ruoyao wrote:
> > > On Mon, 2023-04-17 at 01:33 +0800, WANG Xuerui wrote:
> > > > From: WANG Xuerui <git@xxxxxxxxxx>
> > > >
> > > > Hi,
> > > >
> > > > The LoongArch-64 base architecture is capable of performing
> > > > bounds-checking either before memory accesses or alone, with specialized
> > > > instructions generating BCEs (bounds-checking error) in case of failed
> > > > assertions (ISA manual Volume 1, Sections 2.2.6.1 [1] and 2.2.10.3 [2]).
> > > > This could be useful for managed runtimes, but the exception is not
> > > > being handled so far, resulting in SIGSYSes in these cases, which is
> > > > incorrect and warrants a fix in itself.
> > > >
> > > > During experimentation, it was discovered that there is already UAPI for
> > > > expressing such semantics: SIGSEGV with si_code=SEGV_BNDERR. This was
> > > > originally added for Intel MPX, and there is currently no user (!) after
> > > > the removal of MPX support a few years ago. Although the semantics is
> > > > not a 1:1 match to that of LoongArch, still it is better than
> > > > alternatives such as SIGTRAP or SIGBUS of BUS_OBJERR kind, due to being
> > > > able to convey both the value that failed assertion and the bound value.
> > > >
> > > > This patch series implements just this approach: translating BCEs into
> > > > SIGSEGVs with si_code=SEGV_BNDERR, si_value set to the offending value,
> > > > and si_lower and si_upper set to resemble a range with both lower and
> > > > upper bound while in fact there is only one.
> > > >
> > > > The instructions are not currently used anywhere yet in the fledgling
> > > > LoongArch ecosystem, so it's not very urgent and we could take the time
> > > > to figure out the best way forward (should SEGV_BNDERR turn out not
> > > > suitable).
> > >
> > > I don't think these instructions can be used in any systematic way
> > > within a Linux userspace in 2023.  IMO they should not exist in
> > > LoongArch at all because they have all the same disadvantages of Intel
> > > MPX; MPX has been removed by Intel in 2019, and LoongArch is designed
> > > after 2019.
> >
> > Well, the difference is IMO significant enough to make LoongArch
> > bounds-checking more useful, at least for certain use cases. For
> > example, the bounds were a separate register bank in Intel MPX, but in
> > LoongArch they are just values in GPRs. This fits naturally into
> > JIT-ting or other managed runtimes (e.g. Go) whose slice indexing ops
> > already bounds-check with a temporary register per bound anyway, so it's
> > just a matter of this snippet (or something like it)
> >
> > - calculate element address
> > - if address < base: goto fail
> > - load/calculate upper bound
> > - if address >= upper bound: goto fail
> > - access memory
> >
> > becoming
> >
> > - calculate element address
> > - asrtgt address, base - 1
> > - load/calculate upper bound
> > - {ld,st}le address, upper bound
> >
> > then in SIGSEGV handler, check PC to associate the signal back with the
> > exact access op;
>
> I remember using the signal handler for "usual" error handling can be a
> very bad idea but I can't remember where I've read about it.  Is there
> any managed environments doing so in practice?
>
> If we redefine new_ldle/new_stle as "if [[likely]] the address is in-
> bound, do the load/store and skip the next instruction; otherwise do
> nothing", we can say:
>
> blt        address, base, 1f
> new_ldle.d rd, address, upperbound
> 1:b        panic_oob_access
> xor        rd, rd, 42 // use rd to do something
>
> This is more versatile, and useful for building a loop as well:
>
> or            a0, r0, r0
> 0:new_ldle.d  t1, t0, t2
> b             1f
> add.d         a0, t1, a0
> add.d         t0, t0, 8
> b             0b
> 1:bl          do_something_with_the_sum
>
> Yes it's "non-RISC", but at least more RISC than the current ldle: if
> you want a trap anyway you can say
>
> blt        address, base, 1f
> new_ldle.d rd, address, upperbound
> 1:break    {a code defined for OOB}
> xor        rd, rd, 42 // use rd
>
> --
> Xi Ruoyao <xry111@xxxxxxxxxxx>
> School of Aerospace Science and Technology, Xidian University