mbedtls/tests/scripts/test_zeroize.gdb

# test_zeroize.gdb
#
# This file is part of mbed TLS (https://tls.mbed.org)
#
# Copyright (c) 2017, ARM Limited, All Rights Reserved
#
# Purpose
#
# Run a test using the debugger to check that the mbedtls_zeroize() function in
# utils.h is not being optimized out by the compiler. To do so, the script
# loads the test program at programs/test/zeroize.c and sets a breakpoint at
# the last return statement in the main(). When the breakpoint is hit, the
# debugger manually checks the contents to be zeroized and checks that it is
# actually cleared.
#
# The mbedtls_zeroize() test is debugger driven because there does not seem to
# be a mechanism to reliably check whether the zeroize calls are being
# eliminated by compiler optimizations from within the compiled program. The
# problem is that a compiler would typically remove what it considers to be
# "unecessary" assignments as part of redundant code elimination. To identify
# such code, the compilar will create some form dependency graph between
# reads and writes to variables (among other situations). It will then use this
# data structure to remove redundant code that does not have an impact on the
# program's observable behavior. In the case of mbedtls_zeroize(), an
# intelligent compiler could determine that this function clears a block of
# memory that is not accessed later in the program, so removing the call to
# mbedtls_zeroize() does not have an observable behavior. However, inserting a
# test after a call to mbedtls_zeroize() to check whether the block of
# memory was correctly zeroed would force the compiler to not eliminate the
# mbedtls_zeroize() call. If this does not occur, then the compiler potentially
# has a bug.
#
# Note: This test requires that the test program is compiled with -g3.
#
# WARNING: There does not seem to be a mechanism in GDB scripts to set a
# breakpoint at the end of a function (probably because there are a lot of
# complications as function can have multiple exit points, etc). Therefore, it
# was necessary to hard-code the line number of the breakpoint in the zeroize.c
# test app. The assumption is that zeroize.c is a simple test app that does not
# change often (as opposed to the actual library code), so the breakpoint line
# number does not need to be updated often.

set confirm off
file ./programs/test/zeroize
break zeroize.c:99

set args ./programs/test/zeroize.c
run

set $i = 0
set $len = sizeof(buf)
set $buf = buf

while $i < $len
    if $buf[$i++] != 0
        echo The buffer at was not zeroized\n
        quit 1
    end
end

echo The buffer was correctly zeroized\n

continue

if $_exitcode != 0
    echo The program did not terminate correctly\n
    quit 1
end

quit 0