qsym

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• README.md

Symbolic execution for the QBE IL

qsym is a symbolic execution tool for the QBE intermediate language. The tool leverages Z3 to execute QBE IL based on SMT bitvectors. This enables qsym to reason about conditional jumps in the QBE IL, exploring both branches (if feasible under the current constraints).

Status

qsym is in very early stages of development and presently mostly a proof-of-concept. The underlying parser for the QBE IL (qbe-reader) is also not yet complete, hence it does not support every syntactically valid QBE IL input yet. Furthermore, it is assumed that input programs are well typed, e.g. no type checks are performed for instruction arguments. Simple programs generated using a QBE frontend (e.g. cproc) can already be explored.

Installation

Clone the repository and run the following command:

$ cargo install --path .

Usage Example

Presently, qsym treats the parameters of a selected function as unconstrained symbolic and executes this function. Consider the following example:

$ cat input.qbe
function w $main(w %a) {
@start
        %a =w add 0, %a
        jnz %a, @end1, @end2
@end1
        %exit =w add 0, 1
        hlt
@end2
        %exit =w add 0, 2
        hlt
}
$ qsym input.qbe main
[jnz] Exploring path for label 'end1'
Halting executing
Local variables:
    a = |main:a|
    exit = #x00000001
Symbolic variable values:
    main:a -> #x00000002

[jnz] Exploring path for label 'end2'
Halting executing
Local variables:
    a = |main:a|
    exit = #x00000002
Symbolic variable values:
    main:a -> #x00000000

For the provided example program, qsym discovers two possible execution paths through the function main. In the first execution path the symbolic variable %a is zero, in the other it is non-zero.

License

This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License along with this program. If not, see https://www.gnu.org/licenses/.