Re: [PATCH bpf-next 3/3] selftests/bpf: Disassembler tests for verifier.c:convert_ctx_access()

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-03-03 at 12:28 -0800, Alexei Starovoitov wrote:
> On Fri, Mar 03, 2023 at 12:55:07AM +0200, Eduard Zingerman wrote:
> > Function verifier.c:convert_ctx_access() applies some rewrites to BPF
> > instructions that read or write BPF program context. This commit adds
> > machinery to allow test cases that inspect BPF program after these
> > rewrites are applied.
> > 
> > An example of a test case:
> > 
> >   {
> >         // Shorthand for field offset and size specification
> > 	N(CGROUP_SOCKOPT, struct bpf_sockopt, retval),
> > 
> >         // Pattern generated for field read
> > 	.read  = "$dst = *(u64 *)($ctx + bpf_sockopt_kern::current_task);"
> > 		 "$dst = *(u64 *)($dst + task_struct::bpf_ctx);"
> > 		 "$dst = *(u32 *)($dst + bpf_cg_run_ctx::retval);",
> > 
> >         // Pattern generated for field write
> > 	.write = "*(u64 *)($ctx + bpf_sockopt_kern::tmp_reg) = r9;"
> > 		 "r9 = *(u64 *)($ctx + bpf_sockopt_kern::current_task);"
> > 		 "r9 = *(u64 *)(r9 + task_struct::bpf_ctx);"
> > 		 "*(u32 *)(r9 + bpf_cg_run_ctx::retval) = $src;"
> > 		 "r9 = *(u64 *)($ctx + bpf_sockopt_kern::tmp_reg);" ,
> >   },
> > 
> > For each test case, up to three programs are created:
> > - One that uses BPF_LDX_MEM to read the context field.
> > - One that uses BPF_STX_MEM to write to the context field.
> > - One that uses BPF_ST_MEM to write to the context field.
> > 
> > The disassembly of each program is compared with the pattern specified
> > in the test case.
> > 
> > Kernel code for disassembly is reused (as is in the bpftool).
> > To keep Makefile changes to the minimum, symbolic links to
> > `kernel/bpf/disasm.c` and `kernel/bpf/disasm.h ` are added.
> ...
> > +static regex_t *compile_regex(char *pat)
> > +{
> > +	regex_t *re;
> > +	int err;
> > +
> > +	re = malloc(sizeof(regex_t));
> > +	if (!re) {
> > +		PRINT_FAIL("Can't alloc regex\n");
> > +		return NULL;
> > +	}
> > +
> > +	err = regcomp(re, pat, REG_EXTENDED);
> 
> Fancy.

In a good or in a bad way?
It is the shortest form I came up with...

> What is the cost of running this in test_progs?
> How many seconds does it add to run time?

About 0.13sec (including modprobe and process initialization):

  # time ./test_progs -a "ctx_rewrite/*"
  #58/1    ctx_rewrite/SCHED_CLS.tstamp:OK
  ...
  #58/20   ctx_rewrite/CGROUP_SOCKOPT.optval_end:OK
  #58      ctx_rewrite:OK
  Summary: 1/20 PASSED, 0 SKIPPED, 0 FAILED
  
  real	0m0.131s
  user	0m0.027s
  sys	0m0.046s
  
It loads 52 programs.