Subject: Re: [PATCH] ARM: hw_breakpoint: Handle inexact watchpoint addresses

From: Matthias Kaehlcke <[email-masked]>
Date: Tue, 22 Oct 2019 09:39:21 -0700

On Mon, Oct 21, 2019 at 04:49:48PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Mon, Oct 21, 2019 at 11:47 AM Matthias Kaehlcke <[email-masked]> wrote:
> >
> > On Sat, Oct 19, 2019 at 11:12:26AM -0700, Douglas Anderson wrote:
> > > This is commit fdfeff0f9e3d ("arm64: hw_breakpoint: Handle inexact
> > > watchpoint addresses") but ported to arm32, which has the same
> > > problem.
> > >
> > > This problem was found by Android CTS tests, notably the
> > > "watchpoint_imprecise" test [1].  I tested locally against a copycat
> > > (simplified) version of the test though.
> > >
> > > [1] https://android.googlesource.com/platform/bionic/+/master/tests/sys_ptrace_test.cpp
> > >
> > > Signed-off-by: Douglas Anderson <[email-masked]>
> > > ---
> > >
> > >  arch/arm/kernel/hw_breakpoint.c | 96 ++++++++++++++++++++++++---------
> > >  1 file changed, 70 insertions(+), 26 deletions(-)
> > >
> > > diff --git a/arch/arm/kernel/hw_breakpoint.c b/arch/arm/kernel/hw_breakpoint.c
> > > index b0c195e3a06d..d394878409db 100644
> > > --- a/arch/arm/kernel/hw_breakpoint.c
> > > +++ b/arch/arm/kernel/hw_breakpoint.c
> > > @@ -680,26 +680,62 @@ static void disable_single_step(struct perf_event *bp)
> > >       arch_install_hw_breakpoint(bp);
> > >  }
> > >
> > > +/*
> > > + * Arm32 hardware does not always report a watchpoint hit address that matches
> > > + * one of the watchpoints set. It can also report an address "near" the
> > > + * watchpoint if a single instruction access both watched and unwatched
> > > + * addresses. There is no straight-forward way, short of disassembling the
> > > + * offending instruction, to map that address back to the watchpoint. This
> > > + * function computes the distance of the memory access from the watchpoint as a
> > > + * heuristic for the likelyhood that a given access triggered the watchpoint.
> > > + *
> > > + * See this same function in the arm64 platform code, which has the same
> > > + * problem.
> > > + *
> > > + * The function returns the distance of the address from the bytes watched by
> > > + * the watchpoint. In case of an exact match, it returns 0.
> > > + */
> > > +static u32 get_distance_from_watchpoint(unsigned long addr, u32 val,
> > > +                                     struct arch_hw_breakpoint_ctrl *ctrl)
> > > +{
> > > +     u32 wp_low, wp_high;
> > > +     u32 lens, lene;
> > > +
> > > +     lens = __ffs(ctrl->len);
> >
> > Doesn't this always end up with 'lens == 0'? IIUC ctrl->len can have
> > the values ARM_BREAKPOINT_LEN_{1,2,4,8}:
> >
> > #define ARM_BREAKPOINT_LEN_1    0x1
> > #define ARM_BREAKPOINT_LEN_2    0x3
> > #define ARM_BREAKPOINT_LEN_4    0xf
> > #define ARM_BREAKPOINT_LEN_8    0xff
> 
> Yes, but my best guess without digging into the ARM ARM is that the
> underlying hardware is more flexible.  I don't think it hurts to
> support the flexibility here even if the code creating the breakpoint
> never creates one line that.  ...especially since it makes the arm32
> and arm64 code match in this way.

ok

> > > +     lene = __fls(ctrl->len);
> > > +
> > > +     wp_low = val + lens;
> > > +     wp_high = val + lene;
> >
> > First I thought these values are off by one, but in difference to
> > ffs() from glibc the kernel functions start with index 0, instead
> > of using zero as 'no bit set'.
> 
> Yes, this took me a while.  If you look at the original commit
> fdfeff0f9e3d ("arm64: hw_breakpoint: Handle inexact watchpoint
> addresses") this was clearly done on purpose though.  Specifically
> note the part of the commit message:
> 
> [will: use __ffs instead of ffs - 1]
> 
> 
> > > +     if (addr < wp_low)
> > > +             return wp_low - addr;
> > > +     else if (addr > wp_high)
> > > +             return addr - wp_high;
> > > +     else
> > > +             return 0;
> > > +}
> > > +
> > >  static void watchpoint_handler(unsigned long addr, unsigned int fsr,
> > >                              struct pt_regs *regs)
> > >  {
> > > -     int i, access;
> > > -     u32 val, ctrl_reg, alignment_mask;
> > > +     int i, access, closest_match = 0;
> > > +     u32 min_dist = -1, dist;
> > > +     u32 val, ctrl_reg;
> > >       struct perf_event *wp, **slots;
> > >       struct arch_hw_breakpoint *info;
> > >       struct arch_hw_breakpoint_ctrl ctrl;
> > >
> > >       slots = this_cpu_ptr(wp_on_reg);
> > >
> > > +     /*
> > > +      * Find all watchpoints that match the reported address. If no exact
> > > +      * match is found. Attribute the hit to the closest watchpoint.
> > > +      */
> > > +     rcu_read_lock();
> > >       for (i = 0; i < core_num_wrps; ++i) {
> > > -             rcu_read_lock();
> > > -
> > >               wp = slots[i];
> > > -
> > >               if (wp == NULL)
> > > -                     goto unlock;
> > > +                     continue;
> > >
> > > -             info = counter_arch_bp(wp);
> > >               /*
> > >                * The DFAR is an unknown value on debug architectures prior
> > >                * to 7.1. Since we only allow a single watchpoint on these
> > > @@ -708,33 +744,31 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
> > >                */
> > >               if (debug_arch < ARM_DEBUG_ARCH_V7_1) {
> > >                       BUG_ON(i > 0);
> > > +                     info = counter_arch_bp(wp);
> > >                       info->trigger = wp->attr.bp_addr;
> > >               } else {
> > > -                     if (info->ctrl.len == ARM_BREAKPOINT_LEN_8)
> > > -                             alignment_mask = 0x7;
> > > -                     else
> > > -                             alignment_mask = 0x3;
> > > -
> > > -                     /* Check if the watchpoint value matches. */
> > > -                     val = read_wb_reg(ARM_BASE_WVR + i);
> > > -                     if (val != (addr & ~alignment_mask))
> > > -                             goto unlock;
> > > -
> > > -                     /* Possible match, check the byte address select. */
> > > -                     ctrl_reg = read_wb_reg(ARM_BASE_WCR + i);
> > > -                     decode_ctrl_reg(ctrl_reg, &ctrl);
> > > -                     if (!((1 << (addr & alignment_mask)) & ctrl.len))
> > > -                             goto unlock;
> > > -
> > >                       /* Check that the access type matches. */
> > >                       if (debug_exception_updates_fsr()) {
> > >                               access = (fsr & ARM_FSR_ACCESS_MASK) ?
> > >                                         HW_BREAKPOINT_W : HW_BREAKPOINT_R;
> > >                               if (!(access & hw_breakpoint_type(wp)))
> > > -                                     goto unlock;
> > > +                                     continue;
> > >                       }
> > >
> > > +                     val = read_wb_reg(ARM_BASE_WVR + i);
> > > +                     ctrl_reg = read_wb_reg(ARM_BASE_WCR + i);
> > > +                     decode_ctrl_reg(ctrl_reg, &ctrl);
> > > +                     dist = get_distance_from_watchpoint(addr, val, &ctrl);
> > > +                     if (dist < min_dist) {
> > > +                             min_dist = dist;
> > > +                             closest_match = i;
> > > +                     }
> > > +                     /* Is this an exact match? */
> > > +                     if (dist != 0)
> > > +                             continue;
> > > +
> > >                       /* We have a winner. */
> > > +                     info = counter_arch_bp(wp);
> > >                       info->trigger = addr;
> >
> > Unless we care about using the 'last' watchpoint in case multiple WPs have
> > the same address I think it would be clearer to change the above to:
> >
> >                         if (dist == 0) {
> >                                 /* We have a winner. */
> >                                 info = counter_arch_bp(wp);
> >                                 info->trigger = addr;
> >                                 break;
> >                         }
> 
> Without being an expert on the Hardware Breakpoint API, my
> understanding (based on how the old arm32 code worked and how the
> existing arm64 code works) is that the API accounts for the fact that
> more than one watchpoint can trigger and that we should report on all
> of them.
> 
> Specifically if you do:
> 
> watch 1 byte at 0x1000
> watch 1 byte at 0x1003
> 
> ...and then someone does a single 4-byte write at 0x1000 then both
> watchpoints should trigger.  If we do a "break" here then they won't
> both trigger.

Makes sense, thanks for the example!

> Also note that the triggering happens below in the "perf_bp_event(wp, regs)"
> so with your break I think you'll miss it, no?

You are right, I put that mentally after the loop, we definitely don't
want to skip it :)

> That being said, with my patch we still won't do exactly the right
> thing that for an "imprecise" watchpoint.  Specifically if you do:
> 
> watch 1 byte at 0x1008
> watch 1 byte at 0x100b
> write 16 bytes at 0x1000
> 
> ...then we will _only_ trigger the 0x1008 watchpoint.  ...but that's
> the limitation in how the breakpoints work.  You can see this is what
> happens because the imprecise stuff is outside the for loop and only
> triggers when nothing else did.

It's still an improvement from not triggering at all :)

Not that I'm an expert in this area, but the change looks good to me, so:

Reviewed-by: Matthias Kaehlcke <[email-masked]>

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