If a guard check fails, an error message is printed and the program exits. The guards are initialized when a function is entered and then checked when the function exits. This includes functions that call alloca, and functions with buffers larger than 8 bytes. This is done by adding a guard variable to functions with vulnerable objects.
#Stack smashing detected code
Here is the description of this flag (from the man page) :Įmit extra code to check for buffer overflows, such as stack smashing attacks. While searching for the reason, I came across a gcc flag ‘-fstack-protector’. This prompted me to explore as to how buffer overflow was detected. In the output you can see that stack smashing was detected. Well, this came in as pleasant surprise that the execution environment was somehow able to detect that buffer overflow could happen in this case. This is what happened when I executed the program: $. Since gets() does not check array bounds so it will try to copy the input in the str buffer and this way buffer overflow will take place. The idea here is to input a string whose length is more than 10 bytes. and then calculated the length of this string and printed back on stdout. In the code above, I have used gets() to accept a string from user. Printf("\n len of string entered is : \n", len)
Gets(str) // Used gets() to cause buffer overflow Here is what I was trying to do : #include I came to know about these flags when I was trying to reproduce a buffer overflow on my Ubuntu 12.04 with gcc 4.6.3 version. Earlier it was solely the responsibility of programmers/developers to make sure that there is no possibility of a buffer overflow in their code but with time compilers like gcc have got flags to make sure that buffer overflow problems are not exploited by crackers to damage a system or a program. It refers to attacks that exploit bugs in code enabling buffer overflows. This program should make clear about the error message, in the case of canary value change.Stack smashing is a fancy term used for stack buffer overflows. * The loop is added only to keep gcc happy. _attribute_ ((noreturn)) internal_function In the same GNU library i also found fortify_fail.c #include
So this code simply calls _fortify_fail with argument “smash the stack”. I found this source code form GNU C Library at debug/stack_chk_fail.c Even you can find it from this link _fortify_fail ("stack smashing detected") #includeĮxtern char **_libc_argv attribute_hidden Lets see what does that function contain. if not library routine is beginning called through the PLT.
Than after the “print” operation there is check of canary value that can be seen in line 11, 12 in the case of same value it is moved to 15th line and then return the program normally. When you look at line 5 you could see that some value from global section is moved to $eax and in the next line it is pushed on to the stack. gcc -m32 -fstack-protector-all hello.c -o hello Later i started working on it with a small HelloWorld program which has canary enabled. Recently i was thinking about on how does the canary work in Linux Glibc.