Working with CBMC

This section describes how to access more advanced CBMC options from Kani.

CBMC arguments

Kani is able to handle common CBMC arguments as if they were its own (e.g., --default-unwind <n>), but sometimes it may be necessary to use CBMC arguments which are not handled by Kani.

To pass additional arguments for CBMC, you pass --cbmc-args to Kani. Note that this "switches modes" from Kani arguments to CBMC arguments: Any arguments that appear after --cbmc-args are considered to be CBMC arguments, so all Kani arguments must be placed before it.

Thus, the command line format to invoke cargo kani with CBMC arguments is:

cargo kani [<kani-args>]* --cbmc-args [<cbmc-args>]*

NOTE: In cases where CBMC is not expected to emit a verification output, you have to use Kani's argument --output-format old to turn off the post-processing of output from CBMC.

Individual loop bounds

Setting --default-unwind <n> affects every loop in a harness. Once you know a particular loop is causing trouble, sometimes it can be helpful to provide a specific bound for it.

In the general case, specifying just the highest bound globally for all loops shouldn't cause any problems, except that the solver may take more time because all loops will be unwound to the specified bound.

In situations where you need to optimize for the solver, individual bounds for each loop can be provided on the command line. To do so, we first need to know the labels assigned to each loop with the CBMC argument --show-loops:

# kani src/ --output-format old --cbmc-args --show-loops
Loop _RNvCs6JP7pnlEvdt_3lib17initialize_prefix.0:
  file ./src/ line 11 column 5 function initialize_prefix

Loop _RNvMs8_NtNtCswN0xKFrR8r_4core3ops5rangeINtB5_14RangeInclusivejE8is_emptyCs6JP7pnlEvdt_3lib.0:
  file $RUST/library/core/src/ops/ line 540 column 9 function std::ops::RangeInclusive::<Idx>::is_empty

Loop gen-repeat<[u8; 10]::16806744624734428132>.0:

This command shows us the labels of the loops involved. Note that, as mentioned in CBMC arguments, we need to use --output-format old to avoid post-processing the output from CBMC.

NOTE: At the moment, these labels are constructed using the mangled name of the function and an index. Mangled names are likely to change across different versions, so this method is highly unstable.

Then, we can use the CBMC argument --unwindset label_1:bound_1,label_2:bound_2,... to specify an individual bound for each loop as follows:

kani src/ --cbmc-args --unwindset _RNvCs6JP7pnlEvdt_3lib17initialize_prefix.0:12