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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: Constant Stack Canaries
Date: Sun, 30 Mar 2025 20:14:53 +0000
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On Sun, 30 Mar 2025 17:47:59 +0000, BGB wrote:

> 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.

Agreed.

> Using a magic number

Remove excess words.

> Nothing fancy needed in the assemble or link stages.

They remain blissfully ignorant--at most they generate the magic
number, possibly at random, possibly per link-module.

> 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).
>
> ....