Checking That Functions Are Constant Time with Valgrind (2010)

Posted4 months agoActive4 months ago

g0xA52A2A

11 points

6 comments

imperialviolet.orgTechstory

calmmixed

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40/100

Constant Time FunctionsValgrindSide-Channel Attacks

Key topics

Constant Time Functions

Valgrind

Side-Channel Attacks

The post discusses using Valgrind to check if functions are constant time, with commenters raising concerns about the assumption of constant-time fundamental processor instructions and the age of the blog post.

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Discussion (6 comments)

Showing 6 comments

paulf38

4 months ago

1 reply

That's a 15 year old blog post and the linked GH repo hasn't been updated.

bobmcnamara

4 months ago

It wasn't true back then, but should be common knowledge now: branches have variable timing.

weinzierl

4 months ago

3 replies

"That's assuming that the fundamental processor instructions are constant time, but that's true for all sane CPUs."

Is this really true? Does't the runtime of some arithmetic instructions (like IDIV) depend on the argument?

monkeyelite

4 months ago

There is a small bound for an integer of a given length.

pm215

4 months ago

Yes, especially as CPUs have got faster and put in more clever tricks to improve performance (bear in mind that this blog post is 15 years old). Modern architectures have introduced modes to allow you to turn this off so that for your crypto code you can get back the not-data-dependent timing you wanted. For instance on Arm that's FEAT_DIT:

https://developer.arm.com/documentation/ddi0601/2025-06/AArc...

and the set of instructions you can safely use is listed.

I believe Intel has a similar thing.

quotemstr

4 months ago

Partial solutions are still valuable. This valgrind hack lets you pose the question "Is this code constant time?"

It doesn't tell you "yes" or "no", but it does tell you "definitely not" or "maybe". This information is useful even if it's not the whole story.

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ID: 45351118Type: storyLast synced: 11/20/2025, 5:54:29 PM

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