7 ms·

Unsafe Rust: An Intro and Open Questions

86 points by Ygg2 11y ago

Gankro 11y ago
Author here. I'm currently interning at Mozilla with the goal of writing the "advanced" companion to the TRPL (The Rust Programming Language book): TURPL (The Unsafe Rust Programming Language). In the process I'll need to wrangle various members of the community -- particularly core team members -- to determine the things that we actually intend to guarantee in safe code, and what unsafe code is allowed to "do". This post was intended to kick off that effort by: * Making it clear that stuff is unclear * Asserting my beliefs on what things should be * Getting the whole internet mad at me so that they can explain what it should actually be So, please, single file: Get Mad On The Internet At This Guy
- sanderjd 11y ago
  Thanks for the great article! I think the most important point you make is that unsafety confounds local reasoning, requiring reasoning about all the possible interactions a block of code might have with everything else. One of my favorite things about (safe) Rust (and functional languages in general) is exactly this local reasoning capability that you give up in unsafe code. It's still not clear to me whether or not it's harder to write correct-in-all-cases unsafe Rust code than correct-in-all-cases C/C++/etc. code in general. As I was reading, I was thinking it would be nice for crates.io to include an indication of "level of unsafety" for each crate, but then you went and pointed out that the naive metrics for it would be dependent on stylistic choices. I wonder if you could transform to a "minimally unsafe representation", at either the code or AST level, and evaluate that. Looking forward to TURPL, and especially interested in learning more about how unsafe code interacts with destructors, which seem particularly fraught.
jcranmer 11y ago
Since you mentioned you didn't understand what LLVM's "in bounds" meant: In general, LLVM semantics are largely derived from C and C++ in terms of undefined behavior (although some aspects, like signed overflow, are merely optional). The rule for "in bounds" means that pointers can only be manipulated to point to some address within the bounds of their object, or possibly the address just after the object (which cannot be dereferenced). A concrete example: struct foo { int a; char b; int c[32]; } obj; char ptr = &obj.b; char ptr2 = ptr + kerfuffle; In this example, it is undefined behavior if kerfuffle is not in the range of [-sizeof(int), sizeof(obj) - sizeof(int)]. (e.g., ptr2 == &obj.c[32] is valid, but ptr2 == &obj.c[33] is not).
- kzrdude 11y ago
  That's also this tricky issue with C that's often ignored. If you have a nested array and take a pointer &foo[0][0], the only in bounds pointers are the pointers into the 0th subarray.
  - sunfish 11y ago
    That's true for C, but not for LLVM inbounds.
- Gankro 11y ago
  It was more a jab at LLVM's documentation in general ;) But since you're claiming to understand LLVM docs: * How does LLVM identify that a region of memory is "allocated" per the usage in the GEP docs. In particular it may be useful to mark special addresses as "allocated" for special marker objects that don't actually exist. * Does "in bounds" extend to arrays? e.g. can I offset even further from a ptr to foo if it's in an array of foo?
  - sunfish 11y ago
    "Allocated" is something that arbitrary functions like "malloc" can define for themselves. LLVM specifies what can be done with "allocated memory", and then it's up to the API and implementation of malloc to provide something that works and is useful within LLVM's framework. Dereferencing "allocated" bytes through a proper pointer has to work. Dereferencing "unallocated" bytes is undefined behavior (regardless of whether the deference will succeed or fail in hardware). LLVM's type system is mostly inert in its memory semantics. There is no difference between arrays or any other type of object with respect to what addresses can be computed and dereferenced. The important things are allocations which guarantee contiguous regions of memory.
gregwtmtno 11y ago
This is important for anyone considering using Rust. I was mistaken about the meaning of Rust's safety guarantees before reading this.
- Gankro 11y ago
  Care to elaborate as to what you thought they were?
  - gregwtmtno 11y ago
    Sure. First, thanks for the writeup. I had imagined that the Rust standard library used safe code all the way down. (Whatever that meant, I hadn't put all that much thought into it.) But as you state, "everything is built on top of unsafe." So I guess my understanding after reading this is that I could, using only "safe" code, accidentally manipulate the Rust standard library to cause undefined behavior, it's just much more unlikely in Rust than in C++.
    - Gankro 11y ago
      One important guarantee of Rust though: If you manage to do that, this is a bug in Rust and it's not your fault. And really, that's true of any "safe" language, right? Java, Ruby, Javascript, Python, whatever -- implementation errors mean your program will do crazy bad stuff, and we all have them.
      - dschatz 11y ago
        Just to add to this point, the difference is that Rust can only guarantee this for the standard library. I can similarly write a library with safe interfaces that can be (ab)used to cause UB and there's little that the Rust team can do. This is different from other "safe" languages. This is why it's so important to establish what the responsibility and expectation is of library developers to uphold the safety guarantees that everyone else relies on. It only takes one bad library to destroy the safety guarantees everyone who is transitively using that library relies on.
Animats 11y ago
"Unsafe" is an escape hatch used for a number of reasons. There are good ones and bad ones. Bad ones include: - "I'm so l33t I don't need the compiler to check me." (Don't hire those guys.) - "Safe code is too slow". (File bugs on the compiler's optimizer.) - "Porting this to safe code would require a redesign". (See the Rust port of DOOM.) Most of the real needs for "Unsafe" in Rust come from - The need to interface with external code, including system calls. - Forced type conversion ("casting") - Memory allocation. The first one is mostly a problem with expressive power in the foreign function interface. Can you express what "int read(int fd, char buf[], size_t len)" means in the foreign function definition syntax? Rust's foreign function syntax isn't expressive enough to do that.[1] You can't tell Rust that "len" is the length of "buf". Being able to do that would help reduce the need for unsafe code. Most of the POSIX/Linux API can be described with relatively simple syntax that allows you to associate size info with C arrays. (I once proposed this as an extension to C. It's technically possible but politically too difficult.)[2] If your external interface still requires unsafe code after that, you're probably talking to something that has elaborate foreign data structures visible to the caller. Those really are unsafe. They also usually need a rewrite anyway. (OpenSSL comes to mind.) Forced type conversion, or casting, is traditionally a problem. Most of the trouble comes from C, where casts bypass all type checking. In practice, much casting is safe. If a type is fully mapped to the underlying bits (i.e. all possible bit value are valid for the type), then allowing a cast is safe. If you cast 4 bytes to a 32-bit unsigned integer, the result is always a valid 32-bit unsigned integer. Conversions like that should be explicit, but are not memory-unsafe. On the other hand, casting to a pointer is always unsafe. Again, with a bit more expressive power, the need for unsafe code can be reduced. Memory allocation is hard. However, more of it could be done in safe code. Suppose Rust had a type "space", which is simply an array of bytes, treated as write-only. Constructors take in an array of "space" of the desired type, create a valid local structure with the initialized values, and then perform an operation which copies the structure to the array of "space" and changes its type to the type of the structure. This is safe construction. As an optimization, the compiler can observe that if no reads are made from the local structure prior to converting the "space", the extra local copy is unnecessary. "Space" would still have Rust scope and lifetime, so all that machinery remains hidden. But it's convenient to separate it from construction. Raw memory allocation is complex and unsafe, but separated from the type system, it's a coherent closed system that doesn't get modified much. It's a good candidate for formal proof of correctness - not too big, and critical to system operation. Operations such as expanding vectors seem to include unsafe code. That's worth a hard look. If you had the "space" concept, and the operation that moves a struct into a "safe" array and converts the type, it should be possible to do operations such as growing an array without unsafe code. For Rust 2, it's worth looking at how the need for unsafe code can be reduced. Ultimately, everything should be either memory safe or have a machine proof of memory correctness at the instruction level. [1] https://doc.rust-lang.org/book/ffi.html https://doc.rust-lang.org/book/ffi.html [2] http://www.animats.com/papers/languages/safearraysforc43.pdf http://www.animats.com/papers/languages/safearraysforc43.pdf
TheLoneWolfling 11y ago
> with the caveat that main can't be unsafe Why not? Is there a specific justification for this beyond just "you shouldn't do that"?

86 points by Ygg2 11y ago

Author here. I'm currently interning at Mozilla with the goal of writing the "advanced" companion to the TRPL (The Rust Programming Language book): TURPL (The Unsafe Rust Programming Language). In the process I'll need to wrangle various members of the community -- particularly core team members -- to determine the things that we actually intend to guarantee in safe code, and what unsafe code is allowed to "do". This post was intended to kick off that effort by: * Making it clear that stuff is unclear * Asserting my beliefs on what things should be * Getting the whole internet mad at me so that they can explain what it should actually be So, please, single file: Get Mad On The Internet At This Guy

sanderjd 11y ago
Thanks for the great article! I think the most important point you make is that unsafety confounds local reasoning, requiring reasoning about all the possible interactions a block of code might have with everything else. One of my favorite things about (safe) Rust (and functional languages in general) is exactly this local reasoning capability that you give up in unsafe code. It's still not clear to me whether or not it's harder to write correct-in-all-cases unsafe Rust code than correct-in-all-cases C/C++/etc. code in general. As I was reading, I was thinking it would be nice for crates.io to include an indication of "level of unsafety" for each crate, but then you went and pointed out that the naive metrics for it would be dependent on stylistic choices. I wonder if you could transform to a "minimally unsafe representation", at either the code or AST level, and evaluate that. Looking forward to TURPL, and especially interested in learning more about how unsafe code interacts with destructors, which seem particularly fraught.

jcranmer 11y ago

Since you mentioned you didn't understand what LLVM's "in bounds" meant: In general, LLVM semantics are largely derived from C and C++ in terms of undefined behavior (although some aspects, like signed overflow, are merely optional). The rule for "in bounds" means that pointers can only be manipulated to point to some address within the bounds of their object, or possibly the address just after the object (which cannot be dereferenced). A concrete example: struct foo { int a; char b; int c[32]; } obj; char ptr = &obj.b; char ptr2 = ptr + kerfuffle; In this example, it is undefined behavior if kerfuffle is not in the range of [-sizeof(int), sizeof(obj) - sizeof(int)]. (e.g., ptr2 == &obj.c[32] is valid, but ptr2 == &obj.c[33] is not).

kzrdude 11y ago
That's also this tricky issue with C that's often ignored. If you have a nested array and take a pointer &foo[0][0], the only in bounds pointers are the pointers into the 0th subarray.
- sunfish 11y ago
  That's true for C, but not for LLVM inbounds.
Gankro 11y ago
It was more a jab at LLVM's documentation in general ;) But since you're claiming to understand LLVM docs: * How does LLVM identify that a region of memory is "allocated" per the usage in the GEP docs. In particular it may be useful to mark special addresses as "allocated" for special marker objects that don't actually exist. * Does "in bounds" extend to arrays? e.g. can I offset even further from a ptr to foo if it's in an array of foo?
- sunfish 11y ago
  "Allocated" is something that arbitrary functions like "malloc" can define for themselves. LLVM specifies what can be done with "allocated memory", and then it's up to the API and implementation of malloc to provide something that works and is useful within LLVM's framework. Dereferencing "allocated" bytes through a proper pointer has to work. Dereferencing "unallocated" bytes is undefined behavior (regardless of whether the deference will succeed or fail in hardware). LLVM's type system is mostly inert in its memory semantics. There is no difference between arrays or any other type of object with respect to what addresses can be computed and dereferenced. The important things are allocations which guarantee contiguous regions of memory.

gregwtmtno 11y ago

This is important for anyone considering using Rust. I was mistaken about the meaning of Rust's safety guarantees before reading this.

Gankro 11y ago
Care to elaborate as to what you thought they were?
- gregwtmtno 11y ago
  Sure. First, thanks for the writeup. I had imagined that the Rust standard library used safe code all the way down. (Whatever that meant, I hadn't put all that much thought into it.) But as you state, "everything is built on top of unsafe." So I guess my understanding after reading this is that I could, using only "safe" code, accidentally manipulate the Rust standard library to cause undefined behavior, it's just much more unlikely in Rust than in C++.
  - Gankro 11y ago
    One important guarantee of Rust though: If you manage to do that, this is a bug in Rust and it's not your fault. And really, that's true of any "safe" language, right? Java, Ruby, Javascript, Python, whatever -- implementation errors mean your program will do crazy bad stuff, and we all have them.
    - dschatz 11y ago
      Just to add to this point, the difference is that Rust can only guarantee this for the standard library. I can similarly write a library with safe interfaces that can be (ab)used to cause UB and there's little that the Rust team can do. This is different from other "safe" languages. This is why it's so important to establish what the responsibility and expectation is of library developers to uphold the safety guarantees that everyone else relies on. It only takes one bad library to destroy the safety guarantees everyone who is transitively using that library relies on.

Animats 11y ago

"Unsafe" is an escape hatch used for a number of reasons. There are good ones and bad ones. Bad ones include: - "I'm so l33t I don't need the compiler to check me." (Don't hire those guys.) - "Safe code is too slow". (File bugs on the compiler's optimizer.) - "Porting this to safe code would require a redesign". (See the Rust port of DOOM.) Most of the real needs for "Unsafe" in Rust come from - The need to interface with external code, including system calls. - Forced type conversion ("casting") - Memory allocation. The first one is mostly a problem with expressive power in the foreign function interface. Can you express what "int read(int fd, char buf[], size_t len)" means in the foreign function definition syntax? Rust's foreign function syntax isn't expressive enough to do that.[1] You can't tell Rust that "len" is the length of "buf". Being able to do that would help reduce the need for unsafe code. Most of the POSIX/Linux API can be described with relatively simple syntax that allows you to associate size info with C arrays. (I once proposed this as an extension to C. It's technically possible but politically too difficult.)[2] If your external interface still requires unsafe code after that, you're probably talking to something that has elaborate foreign data structures visible to the caller. Those really are unsafe. They also usually need a rewrite anyway. (OpenSSL comes to mind.) Forced type conversion, or casting, is traditionally a problem. Most of the trouble comes from C, where casts bypass all type checking. In practice, much casting is safe. If a type is fully mapped to the underlying bits (i.e. all possible bit value are valid for the type), then allowing a cast is safe. If you cast 4 bytes to a 32-bit unsigned integer, the result is always a valid 32-bit unsigned integer. Conversions like that should be explicit, but are not memory-unsafe. On the other hand, casting to a pointer is always unsafe. Again, with a bit more expressive power, the need for unsafe code can be reduced. Memory allocation is hard. However, more of it could be done in safe code. Suppose Rust had a type "space", which is simply an array of bytes, treated as write-only. Constructors take in an array of "space" of the desired type, create a valid local structure with the initialized values, and then perform an operation which copies the structure to the array of "space" and changes its type to the type of the structure. This is safe construction. As an optimization, the compiler can observe that if no reads are made from the local structure prior to converting the "space", the extra local copy is unnecessary. "Space" would still have Rust scope and lifetime, so all that machinery remains hidden. But it's convenient to separate it from construction. Raw memory allocation is complex and unsafe, but separated from the type system, it's a coherent closed system that doesn't get modified much. It's a good candidate for formal proof of correctness - not too big, and critical to system operation. Operations such as expanding vectors seem to include unsafe code. That's worth a hard look. If you had the "space" concept, and the operation that moves a struct into a "safe" array and converts the type, it should be possible to do operations such as growing an array without unsafe code. For Rust 2, it's worth looking at how the need for unsafe code can be reduced. Ultimately, everything should be either memory safe or have a machine proof of memory correctness at the instruction level. [1] https://doc.rust-lang.org/book/ffi.html https://doc.rust-lang.org/book/ffi.html [2] http://www.animats.com/papers/languages/safearraysforc43.pdf http://www.animats.com/papers/languages/safearraysforc43.pdf

TheLoneWolfling 11y ago

> with the caveat that main can't be unsafe Why not? Is there a specific justification for this beyond just "you shouldn't do that"?