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Linux kernel library functions formally verified.

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VerKer

Replay Status

To view this file in Russian, please follow the link.

The repository contains ACSL specifications for the Linux kernel functions. The aim of the project is formal verification of Linux kernel library functions.

Papers

Proofs Status

ID Function Status Logic function libfuzzer Comment
1 check_bytes8 proved proved yes
2 match_string proved not required
3 memchr proved yes
4 memcmp proved yes
5 memscan proved not required yes
6 skip_spaces proved proved yes requires too strict (remove strlen)
7 strcasecmp proved yes
8 strcat proved not required usr strcmp in ensures
9 strchr proved proved yes
10 strchrnul proved proved yes
11 strcmp proved proved yes
12 strcpy proved not required use strcmp logic function
13 stpcpy proved not required
14 strcspn proved proved yes
15 strim not required !const
16 strlen proved proved yes
17 strncasecmp yes
18 strncat not required
19 strnchr proved yes
20 strncmp proved yes
21 strncpy not required
22 strnlen proved proved yes
23 strnstr yes
24 strpbrk proved proved yes
25 strrchr proved yes
26 strreplace proved not required !const
27 strsep proved not required !const
28 strspn proved proved yes
29 strstr yes
30 sysfs_streq proved yes
31 strlcat not required
32 strlcpy proved not required use strncmp lf in ensures
33 memmove proved* not required use memcmp logic function at ensures
34 memcpy proved not required use memcmp logic function at ensures
35 memset proved not required !const
36 kstrtobool proved not required yes
37 _parse_integer_fixup_radix proved not required yes
38 _parse_integer yes

* memmove - except pointer difference vc fail. Model limitation.

Toolchain

The specifications are developed in the ACSL language. Frama-C with AstraVer(Jessie) plugin is used as the deductive verification instrument.

  • A description of how to install the tools can be found here. You can run them on Linux, Windows, Mac OS X.
  • By link you can download the VirtualBox VM image in ova format with pre-installed and already configured tools. Image size ~ 3 gigabytes. OS: Ubuntu. Login: user. Password: 1. There are two repositories in the workspace directory. First one is verker, the second is acsl-proved (examples with verification protocols).

How to use the vanilla Frama-C

There is the compatibility layer with the vanilla Frama-C. You can find it in the acsl_syntax_extension.h file. To use the unmodified Frama-C you need to comment out the line #define ASTRAVER_TOOLCHAIN 1 in the acsl syntax extension header. In the Makefile you may want to uncomment FRAMAC line to use the gui version. If you don't want to use the deductive verification plugin you need to redefine FRAMAC_DFLAGS variable in the Makefile.

In details:

diff --git a/acsl_syntax_extension.h b/acsl_syntax_extension.h
index 7018289..ca7b7e3 100644
--- a/acsl_syntax_extension.h
+++ b/acsl_syntax_extension.h
@@ -5,7 +5,7 @@
  * Comment this define if you want to use
  * vanilla Frama-C.
  */
-#define ASTRAVER_TOOLCHAIN 1
+//#define ASTRAVER_TOOLCHAIN 1
 
 
 #ifdef ASTRAVER_TOOLCHAIN /* used by default */
diff --git a/Makefile b/Makefile
index 4c6f529..0567e01 100644
--- a/Makefile
+++ b/Makefile
@@ -21,12 +21,12 @@ OPAM_EVAL        := eval $$(opam env)
 else
 OPAM_EVAL        := eval $$(opam config env)
 endif
-#FRAMAC           := $(OPAM_EVAL); frama-c-gui -c11 -cpp-extra-args " -C -E -x c $(SPEC_CFLAGS) " -machdep gcc_x86_64
-FRAMAC           := $(OPAM_EVAL); frama-c -c11 -cpp-extra-args " -C -E -x c $(SPEC_CFLAGS) " -machdep gcc_x86_64
+FRAMAC           := $(OPAM_EVAL); frama-c-gui -c11 -cpp-extra-args " -C -E -x c $(SPEC_CFLAGS) " -machdep gcc_x86_64
+#FRAMAC           := $(OPAM_EVAL); frama-c -c11 -cpp-extra-args " -C -E -x c $(SPEC_CFLAGS) " -machdep gcc_x86_64
 FRAMAC_NOHUP     := $(OPAM_EVAL); nohup frama-c -c11 -cpp-extra-args " -C -E -x c $(SPEC_CFLAGS) " -machdep gcc_x86_64
-#FRAMAC_DFLAGS    :=
+FRAMAC_DFLAGS    :=
 #FRAMAC_DFLAGS    := -wp
-FRAMAC_DFLAGS    := -av
+#FRAMAC_DFLAGS    := -av
 FRAMAC_UFLAGS    := -av -av-target update
 FRAMAC_REPLAY    := -av-target why3autoreplay
 FRAMAC_SPROVE    := -av-target why3sprove -av-why3-opt " --strategy proof_juicer --theory-filter axiom"

Repository files

Each library function of the Linux kernel is located in a separate *.c file. The corresponding *.h file contains declarations, types, and structures specific to the function.

  • The kernel_definitons.h file contains common for all functions types, macros, and other declarations.
  • In ctype.h there are several functions, which were initially developed as macro. For the convenience of formal verification, these macro (islower, isupper, isdigit, ...) have been rewritten as an inline functions.
  • The acsl_syntax_extension.h file contains the compatibility layer for the vanilla Frama-C.

How to run

You can type make help to see the available options. It's recommended to start with already proved functions.

Proofs Replay

To replay the proofs on your PC you need to type make replay-proved. If you type make replay-proved-separatedly the instrument will try to replay proofs for functions one-by-one. CVC-1.4, CVC-1.6, Alt-Ergo-2.2.0 solvers are required to replay proofs.

Prove

To run the tools on a particular file you need to type make verify-<function>. The command will run the Why3 ide with a number of verification conditions. If you type make verify-proved the instrument will be run only on the already proved functions.

How to add a function in the repository

There is a tool called dismember in the repository. It is used to "transfer" the function code into a separate file. Example (code for the strim function):

$ dismember -m ~/linux-stable/lib/string.c -k ~/linux-stable --double -f strim --output-dir .

Two files will be created: strim.c and strim.h

  • -m - path to the file with function definition
  • -k - path to the kernel directory
  • -double - generate two files *.h and *.c
  • -f - function name
  • -output-dir - output directory

Specifications

The specification contract (precondition and postcondition) is located in the corresponding *.h file for each proved function (for example, strlen.h). A header file may also contain lemmas/axioms/logical functions if they are developed for a function.

A *.c file contain a body of a function with loops invariants, evaluation functions, and assertions.

For some functions, specifications are redundant. In fact, they describe function's behavior in two different ways. For example, the contract for the strlen function consists of a "regular" functional requirements and the requirement for correspondence of the returned result to the logical function strlen.

What is the reason for a such "redundancy"?

The logical function strlen is convenient to use in the other function's specification. For example, strcmp function (and strcmp logical function in the strcpy contract). All the basic properties of a logical functions are expressed in lemmas (lemmas are not proved at this stage). Such specifications can't be translated in the run-time assertions with E-ACSL plugin. Therefore, for those functions with a correspondent logical function, there are additionally exists a "usual" specification.

Criteria to develop a logical function:

  1. It is possible to write a logical function only for a pure C function;
  2. It is rational to write logical functions if they are useful for developing specifications of other functions. For example, in the memcpy contract, you can express the equality of src and dest by calling the memcmp logical function.

Full verification protocols (solver launches) are included (*.av folders) in the repository for the proved funtions.

At the given stage, the correctness of the lemmas in the specifications is not proved in any way. Thus, they can contain contradictions. The lemmas will be proved at the second project stage by means of Coq or lemma functions.

How to run the instruments

$ frama-c -av <func>.c
$ frama-c -av check_bytes8.c

Or

$ make verify-<func>
$ make verify-check_bytes8

LibFuzzer integration

LibFuzzer - is the library for function fuzzing. The status of functions fuzzing integration can be viewed in Proofs Status table. It is required to have clang compiler installed and libFuzzer.a in the project directory to run fuzzing. How to run fuzzing:

$ make fuzz-<func>
$ make fuzz-check_bytes8

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