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From: BGB <cr88192@gmail.com>
Newsgroups: comp.arch
Subject: Re: Constant Stack Canaries
Date: Sun, 30 Mar 2025 12:47:59 -0500
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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).

....