selftests/bpf: BPF register range bounds tester

Add test to validate BPF verifier's register range bounds tracking logic.

The main bulk is a lot of auto-generated tests based on a small set of
seed values for lower and upper 32 bits of full 64-bit values.
Currently we validate only range vs const comparisons, but the idea is
to start validating range over range comparisons in subsequent patch set.

When setting up initial register ranges we treat registers as one of
u64/s64/u32/s32 numeric types, and then independently perform conditional
comparisons based on a potentially different u64/s64/u32/s32 types. This
tests lots of tricky cases of deriving bounds information across
different numeric domains.

Given there are lots of auto-generated cases, we guard them behind
SLOW_TESTS=1 envvar requirement, and skip them altogether otherwise.
With current full set of upper/lower seed value, all supported
comparison operators and all the combinations of u64/s64/u32/s32 number
domains, we get about 7.7 million tests, which run in about 35 minutes
on my local qemu instance without parallelization. But we also split
those tests by init/cond numeric types, which allows to rely on
test_progs's parallelization of tests with `-j` option, getting run time
down to about 5 minutes on 8 cores. It's still something that shouldn't
be run during normal test_progs run.  But we can run it a reasonable
time, and so perhaps a nightly CI test run (once we have it) would be
a good option for this.

We also add a small set of tricky conditions that came up during
development and triggered various bugs or corner cases in either
selftest's reimplementation of range bounds logic or in verifier's logic
itself. These are fast enough to be run as part of normal test_progs
test run and are great for a quick sanity checking.

Let's take a look at test output to understand what's going on:

  $ sudo ./test_progs -t reg_bounds_crafted
  #191/1   reg_bounds_crafted/(u64)[0; 0xffffffff] (u64)< 0:OK
  ...
  #191/115 reg_bounds_crafted/(u64)[0; 0x17fffffff] (s32)< 0:OK
  ...
  #191/137 reg_bounds_crafted/(u64)[0xffffffff; 0x100000000] (u64)== 0:OK

Each test case is uniquely and fully described by this generated string.
E.g.: "(u64)[0; 0x17fffffff] (s32)< 0". This means that we
initialize a register (R6) in such a way that verifier knows that it can
have a value in [(u64)0; (u64)0x17fffffff] range. Another
register (R7) is also set up as u64, but this time a constant (zero in
this case). They then are compared using 32-bit signed < operation.
Resulting TRUE/FALSE branches are evaluated (including cases where it's
known that one of the branches will never be taken, in which case we
validate that verifier also determines this as a dead code). Test
validates that verifier's final register state matches expected state
based on selftest's own reg_state logic, implemented from scratch for
cross-checking purposes.

These test names can be conveniently used for further debugging, and if -vv
verboseness is requested we can get a corresponding verifier log (with
mark_precise logs filtered out as irrelevant and distracting). Example below is
slightly redacted for brevity, omitting irrelevant register output in
some places, marked with [...].

  $ sudo ./test_progs -a 'reg_bounds_crafted/(u32)[0; U32_MAX] (s32)< -1' -vv
  ...
  VERIFIER LOG:
  ========================
  func#0 @0
  0: R1=ctx(off=0,imm=0) R10=fp0
  0: (05) goto pc+2
  3: (85) call bpf_get_current_pid_tgid#14      ; R0_w=scalar()
  4: (bc) w6 = w0                       ; R0_w=scalar() R6_w=scalar(smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
  5: (85) call bpf_get_current_pid_tgid#14      ; R0_w=scalar()
  6: (bc) w7 = w0                       ; R0_w=scalar() R7_w=scalar(smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
  7: (b4) w1 = 0                        ; R1_w=0
  8: (b4) w2 = -1                       ; R2=4294967295
  9: (ae) if w6 < w1 goto pc-9
  9: R1=0 R6=scalar(smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
  10: (2e) if w6 > w2 goto pc-10
  10: R2=4294967295 R6=scalar(smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
  11: (b4) w1 = -1                      ; R1_w=4294967295
  12: (b4) w2 = -1                      ; R2_w=4294967295
  13: (ae) if w7 < w1 goto pc-13        ; R1_w=4294967295 R7=4294967295
  14: (2e) if w7 > w2 goto pc-14
  14: R2_w=4294967295 R7=4294967295
  15: (bc) w0 = w6                      ; [...] R6=scalar(id=1,smin=0,smax=umax=4294967295,var_off=(0x0; 0xffffffff))
  16: (bc) w0 = w7                      ; [...] R7=4294967295
  17: (ce) if w6 s< w7 goto pc+3        ; R6=scalar(id=1,smin=0,smax=umax=4294967295,smin32=-1,var_off=(0x0; 0xffffffff)) R7=4294967295
  18: (bc) w0 = w6                      ; [...] R6=scalar(id=1,smin=0,smax=umax=4294967295,smin32=-1,var_off=(0x0; 0xffffffff))
  19: (bc) w0 = w7                      ; [...] R7=4294967295
  20: (95) exit

  from 17 to 21: [...]
  21: (bc) w0 = w6                      ; [...] R6=scalar(id=1,smin=umin=umin32=2147483648,smax=umax=umax32=4294967294,smax32=-2,var_off=(0x80000000; 0x7fffffff))
  22: (bc) w0 = w7                      ; [...] R7=4294967295
  23: (95) exit

  from 13 to 1: [...]
  1: [...]
  1: (b7) r0 = 0                        ; R0_w=0
  2: (95) exit
  processed 24 insns (limit 1000000) max_states_per_insn 0 total_states 2 peak_states 2 mark_read 1
  =====================

Verifier log above is for `(u32)[0; U32_MAX] (s32)< -1` use cases, where u32
range is used for initialization, followed by signed < operator. Note
how we use w6/w7 in this case for register initialization (it would be
R6/R7 for 64-bit types) and then `if w6 s< w7` for comparison at
instruction #17. It will be `if R6 < R7` for 64-bit unsigned comparison.
Above example gives a good impression of the overall structure of a BPF
programs generated for reg_bounds tests.

In the future, this "framework" can be extended to test not just
conditional jumps, but also arithmetic operations. Adding randomized
testing is another possibility.

Some implementation notes. We basically have our own generics-like
operations on numbers, where all the numbers are stored in u64, but how
they are interpreted is passed as runtime argument enum num_t. Further,
`struct range` represents a bounds range, and those are collected
together into a minimal `struct reg_state`, which collects range bounds
across all four numberical domains: u64, s64, u32, s64.

Based on these primitives and `enum op` representing possible
conditional operation (<, <=, >, >=, ==, !=), there is a set of generic
helpers to perform "range arithmetics", which is used to maintain struct
reg_state. We simulate what verifier will do for reg bounds of R6 and R7
registers using these range and reg_state primitives. Simulated
information is used to determine branch taken conclusion and expected
exact register state across all four number domains.

Implementation of "range arithmetics" is more generic than what verifier
is currently performing: it allows range over range comparisons and
adjustments. This is the intended end goal of this patch set overall and verifier
logic is enhanced in subsequent patches in this series to handle range
vs range operations, at which point selftests are extended to validate
these conditions as well. For now it's range vs const cases only.

Note that tests are split into multiple groups by their numeric types
for initialization of ranges and for comparison operation. This allows
to use test_progs's -j parallelization to speed up tests, as we now have
16 groups of parallel running tests. Overall reduction of running time
that allows is pretty good, we go down from more than 30 minutes to
slightly less than 5 minutes running time.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>