Re: [PATCH bpf-next 2/3] bpf: selftests: Add bpf_assert_if() and bpf_assert_with_if() macros

[Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>]

On Thu, Dec 14, 2023 at 6:46 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Dec 14, 2023 at 2:56 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
> >
> > These macros are a temporary stop-gap until bpf exceptions support
> > unwinding acquired entities. Basically these macros act as if they take
> > a callback which only get executed if the assertion fails.
> >
> > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > ---
> >  .../testing/selftests/bpf/bpf_experimental.h  | 22 +++++++++++++++++++
> >  1 file changed, 22 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
> > index 1386baf9ae4a..d63f415bef26 100644
> > --- a/tools/testing/selftests/bpf/bpf_experimental.h
> > +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> > @@ -263,6 +263,17 @@ extern void bpf_throw(u64 cookie) __ksym;
> >   */
> >  #define bpf_assert(cond) if (!(cond)) bpf_throw(0);
> >
> > +/* Description
> > + *     Assert that a conditional expression is true. If false, runs code in the
> > + *     body before throwing.
> > + * Returns
> > + *     Void.
> > + * Throws
> > + *     An exception with the value zero when the assertion fails.
> > + */
> > +#define bpf_assert_if(cond) \
> > +       for (int ___i = 0, ___j = !!(cond); !(___j) && !___i; bpf_throw(0), ___i++)
>
> Kumar,
>
> Is this approach reliable?
> I suspect the compiler can still optimize it.
> I feel it will be annoying to clean up if folks start using it now,
> since there won't be a drop in replacement.
> Every such bpf_assert_if() would need to be manually patched.
>
> If 2nd part of exception is far, how about we add an equivalent
> of __bpf_assert() macroses with conditional ops in asm,
> but with extra 'asm volatile goto' that can be used to construct
> release of resources.
>
> bpf_do_assert_eq(var1, 0) { bpf_spin_unlock(...); }
> bpf_do_assert_lt(var2, 0) { bpf_spin_unlock(...); }

Just realized that we can go the other way instead.

We can get rid of bpf_assert_eq/ne/... and replace with:

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h
b/tools/testing/selftests/bpf/bpf_experimental.h
index 1386baf9ae4a..1c500287766d 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -254,6 +254,15 @@ extern void bpf_throw(u64 cookie) __ksym;
                }
                 \
         })

+#define _EQ(LHS, RHS) \
+       ({ int var = 1;\
+               asm volatile goto("if %[lhs] == %[rhs] goto %l[l_yes]" \
+               :: [lhs] "r"(LHS), [rhs] "i"(RHS) :: l_yes);\
+       var = 0;\
+l_yes:\
+       var;\
+       })
+
 /* Description
  *     Assert that a conditional expression is true.
  * Returns
diff --git a/tools/testing/selftests/bpf/progs/exceptions.c
b/tools/testing/selftests/bpf/progs/exceptions.c
index 2811ee842b01..1111e852f154 100644
--- a/tools/testing/selftests/bpf/progs/exceptions.c
+++ b/tools/testing/selftests/bpf/progs/exceptions.c
@@ -203,6 +203,7 @@ __noinline int assert_nz_gfunc(u64 c)
        volatile u64 cookie = c;

        bpf_assert(cookie != 0);
+       bpf_assert(_EQ(cookie, 2));
        return 0;
 }

we can probably remove bpf_assert_with() and
all of the bpf_assert_le|ne|qt|eq|_with()

Users can open code everything:
if (!_EQ(foo, bar)) bpf_throw(123);

bpf_assert_if() can work too,
but let's call it bpf_do_assert() and use like:

bpf_do_assert(EQ(time, 0)) {
   // cleanup
}