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[web.git] / ralf / _posts / 2017-07-14-undefined-behavior.md

diff --git a/ralf/_posts/2017-07-14-undefined-behavior.md b/ralf/_posts/2017-07-14-undefined-behavior.md
index 851f737909324a3cb7d6d5d710ece80e84f44e88..26171ac992ea597aafd92d74ee38cd3b252f044e 100644 (file)
--- a/ralf/_posts/2017-07-14-undefined-behavior.md
+++ b/ralf/_posts/2017-07-14-undefined-behavior.md
@@ -62,6 +62,8 @@ Unfortunately, it is often not easy to say whether a program has undefined behav
 Furthermore, while C compilers are happy to exploit the fact that a particular program *has* UB, they do not provide a way to test that executing a program *does not* trigger UB.
 It also turns out that programmers' intuition often [does not match what the compiler does](https://www.cl.cam.ac.uk/~pes20/cerberus/notes50-survey-discussion.html), which leads to miscompilations (in the eye of the programmer) and sometimes to security [vulnerabilities](https://lwn.net/Articles/342330/).
 As a consequence, UB has a pretty bad reputation.
 Furthermore, while C compilers are happy to exploit the fact that a particular program *has* UB, they do not provide a way to test that executing a program *does not* trigger UB.
 It also turns out that programmers' intuition often [does not match what the compiler does](https://www.cl.cam.ac.uk/~pes20/cerberus/notes50-survey-discussion.html), which leads to miscompilations (in the eye of the programmer) and sometimes to security [vulnerabilities](https://lwn.net/Articles/342330/).
 As a consequence, UB has a pretty bad reputation.

+(The fact that most people will not expect an innocent-looking `+` operation to come with subtle proof obligations concerning overflow probably also plays a role in this.
+In other words, this is also an API design problem.)

 
 There are various sanitizers that watch a program while it is being executed and try to detect UB, but they are not able to catch all possible sources of UB.
 Part of the reason this is so hard is that the standard has not been written with such sanitizers in mind.
 
 There are various sanitizers that watch a program while it is being executed and try to detect UB, but they are not able to catch all possible sources of UB.
 Part of the reason this is so hard is that the standard has not been written with such sanitizers in mind.


@@ -72,7 +74,7 @@ That is not very satisfying.
 ## Undefined Behavior in Rust
 
 Coming back to Rust, where are we at?
 ## Undefined Behavior in Rust
 
 Coming back to Rust, where are we at?

-Safe Rust is [free from UB]({{ site.baseurl }}{% post_url 2017-07-08-rustbelt %}), but we still have to worry about unsafe Rust.
+Safe Rust is [free from UB]({% post_url 2017-07-08-rustbelt %}), but we still have to worry about unsafe Rust.

 For example, what if unsafe code crafts two aliasing mutable references (something that is prevented in safe Rust) and passes them to our `simple` function?
 This violates the assumptions we made when we reordered the two writes.
 If we want to permit this optimization (which we do!), we have to argue why it cannot change program behavior.
 For example, what if unsafe code crafts two aliasing mutable references (something that is prevented in safe Rust) and passes them to our `simple` function?
 This violates the assumptions we made when we reordered the two writes.
 If we want to permit this optimization (which we do!), we have to argue why it cannot change program behavior.


@@ -89,11 +91,11 @@ I also think that tooling to *detect* UB is of paramount importance, and can hel
 To this end, the specification should be written in a way that such tooling is feasible.
 In fact, specifying a dynamic UB checker is a very good way to specify UB!
 Such a specification would describe the additional state that is needed at run-time to then *check* at every operation whether we are running into UB.
 To this end, the specification should be written in a way that such tooling is feasible.
 In fact, specifying a dynamic UB checker is a very good way to specify UB!
 Such a specification would describe the additional state that is needed at run-time to then *check* at every operation whether we are running into UB.

-It is with such considerations in my mind that I have previously written about [miri as an executable specification]({{ site.baseurl }}{% post_url 2017-06-06-MIR-semantics %}).
+It is with such considerations in my mind that I have previously written about [miri as an executable specification]({% post_url 2017-06-06-MIR-semantics %}).

 
 

-Coming up next on this channel:  During my [internship]({{ site.baseurl }}{% post_url 2017-05-23-internship-starting %}), I am working on such a specification.
+Coming up next on this channel:  During my [internship]({% post_url 2017-05-23-internship-starting %}), I am working on such a specification.

 My ideas are concrete enough now that I can write down a draft, which I will share with the world to see what the world thinks about it.
 
 My ideas are concrete enough now that I can write down a draft, which I will share with the world to see what the world thinks about it.
 

-**Update:** [Writing down has happened]({{ site.baseurl }}{% post_url 2017-07-17-types-as-contracts %}).
+**Update:** [Writing down has happened]({% post_url 2017-07-17-types-as-contracts %}).

 
 **Update:** Clarified "Shifting Responsibility".
 
 **Update:** Clarified "Shifting Responsibility".