Re: Constant Stack Canaries

On 3/30/2025 7:16 AM, Robert Finch wrote:

Just got to thinking about stack canaries. I was going to have a special purpose register holding the canary value for testing while the program was running. But I just realized today that it may not be needed. Canary values could be handled by the program loader as constants, eliminating the need for a register. Since the value is not changing while the program is running, it could easily be a constant. This may require a fixup record handled by the assembler / linker to indicate to the loader to place a canary value.
 Prolog code would just store an immediate to the stack. On return a TRAP instruction could check for the immediate value and trap if not present.
But the process seems to require assembler / linker support.
 

They are mostly just a normal compiler feature IME:
   Prolog stores the value;
   Epilog loads it and verifies that the value is intact.
Using a magic number generated by the compiler.
Nothing fancy needed in the assemble or link stages.
In my case, canary behavior is one of:
   Use them in functions with arrays or similar (default);
   Use them everywhere (optional);
   Disable them entirely (also optional).
In my case, it is only checking 16-bit magic numbers, but mostly because a 16-bit constant is cheaper to load into a register in this case (single 32-bit instruction, vs a larger encoding needed for larger values).
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