11 ms·

This is great! The WebAssembly Core Specification is actually quite readable, although some of the language can be a bit intimidating if you're not used to read

This is great! The WebAssembly Core Specification is actually quite readable, although some of the language can be a bit intimidating if you're not used to reading programming language papers. If anyone is looking for a slightly more accessible way to learn WebAssembly, you might enjoy WebAssembly from the Ground Up: https://wasmgroundup.com https://wasmgroundup.com (Disclaimer: I'm one of the authors)

MuffinFlavored 2y ago
I know one of WebAssembly's biggest features by design is security / "sandbox". But I've always gotten confused with... it is secure because by default it can't do much. I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter. I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa?
- jeroenhd 2y ago
  WebAssembly can communicate through buffers. WebAssembly can also import foreign functions (Javascript functions in the browser). You can get output by reading the buffer at the end of execution/when receiving callbacks. So, for instance, you pass a few frames worth of buffers to WASM, WASM renders pixels into the buffers, calls a callback, and the Javascript reads data from the buffer (sending it to a <canvas> or similar). The benefit of WASM is that it can't be very malicious by itself. It requires the runtime to provide it with exported functions and callbacks to do any file I/O, network I/O, or spawning new tasks. Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to, while WASM can only interact with the specific API surface you provide it. That makes WASM less powerful, but more predictable, and in my opinion better for building integrations with as there is no risk of internal APIs being accessed (that you will be blamed for if they break in an update).
  - panic 2y ago
    I don’t believe it is currently possible for a WebAssembly instance to access any buffer other than its own memory. You have to copy data in and out.
    - deathanatos 2y ago
      The embedder could hand the module functions for manipulating external buffers via externrefs. (I'm not sure if that's a good idea, or not, just that it could.) But if the module wants to compute on the values in the buffer, at some level it would have to copy the data in/out.
    - davexunit 2y ago
      Use the GC instructions and you can freely share heap references amongst other modules and the host.
      - panic 2y ago
        How do you access the contents of a heap reference from JavaScript in order to “send it to a <canvas> or similar”?
        davexunit 2y ago
        Assuming you're talking about reading binary data like (array i8), the GC MVP doesn't have a great answer right now. Have to call back into wasm to read the bytes. Something for the group to address in future proposals. Sharing between wasm modules is better right now.
  - brabel 2y ago
    > Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to That's not correct, when you embed Lua you can choose which APIs are available, to make the full stdlib available you must explicitly call `luaL_openlibs` [1]. [1] https://www.lua.org/manual/5.3/manual.html#luaL_openlibs https://www.lua.org/manual/5.3/manual.html#luaL_openlibs
- Karellen 2y ago
  > You write in one language Not quite. Web assembly isn't a source language, it's a compiler target. So you should be able to write in C, Rust, Fortran, or Lua and compile any of those to WebAssembly. Except that WebAssembly is a cross-platform assembly language/machine code which is very similar to the native machine code of many/most contemporary CPUs. This means a WebAssembly interpreter can be very straightforward, and could often translate one WebAssembly instruction to one native CPU instruction. Or rather, it can compile a stream of WebAssembly instructions almost one-to-one to native CPU instructions, which it can then execute directly.
  - whizzter 2y ago
    A JIT should be able to translate most arithmetic and binary instructions to single-opcodes, however anything involving memory and functions calls needs safety checks that becomes multi-instruction. branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops (if it also wants to be crash-safe even if it's exploit safe).
    - kouteiheika 2y ago
      > anything involving memory [..] needs safety checks that becomes multi-instruction Not necessarily; on AMD64 you can do memory accesses in a single instruction relatively easily by using the CPU's paging machinery for safety checks plus some clever use of address space. > branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops Even with metering the branches would be direct, you'd just insert the metering code at the start of each basic block (so that's two extra instructions at the start of each basic block). Or did you mean something else?
      - charleslmunger 2y ago
        Metering also doesn't require a branch if you implement it with page faults. See "Implicit suspend checks" in https://android-developers.googleblog.com/2023/11/the-secret-to-androids-improved-memory-latest-android-runtime-update.html?m=1 https://android-developers.googleblog.com/2023/11/the-secret...
- pdubroy 2y ago
  You should check out the book :-) We have a chapter called "What Makes WebAssembly Safe?" which covers the details. You can get a sneak peek here: https://bsky.app/profile/wasmgroundup.com/post/3lh2e4eiwnm2p https://bsky.app/profile/wasmgroundup.com/post/3lh2e4eiwnm2p
- coliveira 2y ago
  I think the biggest advantage of wasm in terms of security is that it doesn't accept machine language written in the target machine, only in this artificial machine language. This means that it cannot encode arbitrary code that could be executed by the host machine. Everything it runs has necessarily to go through the wasm interpreter.
  - hb-robo 2y ago
    That's quite interesting. This is way outside of my wheelhouse - has this kind of approach been tried in other security contexts before? What would you even call that, virtualization?
    - tubs 2y ago
      Java.
    - dmitrygr 2y ago
      The word is "bytecode" and the idea is as old as computing.
  - wyldfire 2y ago
    > This means that it cannot encode arbitrary code that could be executed by the host machine. But the host machine still can, so it's not as big of advantage in that regard. If you could somehow deliver a payload of native code and jump to it, it'd work just fine. But the security you get is the fact that it's really hard to do that because there's no wasm instructions to jump to arbitrary memory locations (even if all the host ISAs do have those). Having a VM alone doesn't provide security against attacks. It's often the case that VMs are used with memory-safe languages and those languages' runtime bounds checks and other features are what gives them safety moreso than their VM. In fact, most bytecode languages provide a JIT (including some wasm deployments) so you're actually running native code regardless.
- Gupta2 2y ago
  Speaking of WebAssembly security, is it vulnerable to Spectre/CPU style attacks like those in JavaScript? (WASM without imported JS functions)
  - saagarjha 2y ago
    Yes.
  - jeffparsons 2y ago
    Yes, if you give the Wasm instance access to timers.
- pizlonator 2y ago
  > But I've always gotten confused with... it is secure because by default it can't do much. Yes. That’s a super accurate description. You’re not confused. > I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter. Almost. Wasm is cheap to JIT compile and the resulting code is usually super efficient. Sometimes parity with native execution. > I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa? It’s definitely a case where the investment:utility ratio is high. ;-) Here’s the trade off between embedding Lua and embedding Wasm: - Both have the problem that they are only as secure as the API you expose to the guest program. If you expose `rm -rf /` to either Lua or Wasm, you’ll have a bad time. And it’s surprisingly difficult to convince yourself that you didn’t accidentally do that. Security is hard. - Wasm is faster than Lua. - Lua is a language for humans, no need for another language and compiler. That makes Lua a more natural choice for embedded scripting. - Lua is object oriented, garbage collected, and has a very principled story for how that gets exposed to the host in a safe way. Wasm source languages are usually not GC’d. That means that if you want to expose object oriented API to the guest program, then it’ll feel more natural to do that with Lua. - The wasm security model is dead simple and doesn’t (necessarily) rely on anything like GC, making it easier to convince yourself that the wasm implementation is free of security vulnerabilities. If you want a sandboxed execution environment then Wasm is better for that reason.
- jedisct1 2y ago
  It's like JavaScript, Python or PHP. There are no pointers, only arrays, that cause the app to crash if it attempts to read out of the bounds.
veltas 2y ago
> actually quite readable, although some of the language can be a bit intimidating if you're not used to reading programming language papers You're more generous than me, I think it's rubbish. Would have been easier to read if they had written it more like an ISA manual.
- amw-zero 2y ago
  This is an opportunity to learn. The way WebAssembly is defined is the standard way PL semantics are defined.
  - veltas 2y ago
    WebAssembly isn't a programming language, it's an ISA or a bytecode VM. They've defined it in a very over-engineered way, with different representations all the way through. It has created issues for people working on assemblers and code generation because it's hard to choose a good representation for compilers to generate. This all could have been avoided.
    - amw-zero 2y ago
      Those are all programming languages.
- mananaysiempre 2y ago
  You can understand the WASM spec in your sleep if you’ve ever worked through a type-system paper from the last two decades (or a logic paper from even earlier I guess). Granted, not many people have, but there’s a reason why it makes sense for it to be written in that style: they want it to be very clear that the verification (typechecking, really) algorithm doesn’t have any holes, and for that it’s reasonable to speak the language of the people who prove that type of thing for a living. The WASM spec is also the ultimate authoritative reference for both programmers and implementers. That’s different from the goals of an ISA manual, which usually only targets programmers and just says “don’t do that” for certain dark corners of the (sole) implementation. (The RISC-V manual is atypical in this respect; still, I challenge you to describe e.g. which PC value the handler will see if the user code traps on a base RV32IMA system.)
deleted 2y ago
[deleted]
amw-zero 2y ago
I think it's much better to just learn how to read inference rules. They're actually quite simple, and are used ubiquitously to define PL semantics definitions. Constraining this on "that's not an option" is a big waste of time - learning this will open up all of the literature written on the subject.
- shpongled 2y ago
  The WASM spec is so well defined presumably because Andreas Rossberg is the editor - and he did a bunch of PL research on extensions to Standard ML, which is famous for it's specification!
  - amw-zero 2y ago
    I totally agree. Standard ML set the stage for everyone to finally formally specify a full language, and only WebAssembly has carried the torch as far as I know (other than small research languages).
crabmusket 2y ago
I bought the early access of your book a while ago and completed the first few chapters which were available. I found it a fantastic resource, and I was keen to continue but other responsibilities have gotten in the way since. I recommend it!
- pdubroy 2y ago
  Glad to hear! If you haven't looked at it in a while — we just published a draft of the final technical chapter, and are planning an official launch on March 4. So, might be a good time to dig back in :-)

MuffinFlavored 2y ago

I know one of WebAssembly's biggest features by design is security / "sandbox". But I've always gotten confused with... it is secure because by default it can't do much. I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter. I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa?

jeroenhd 2y ago
WebAssembly can communicate through buffers. WebAssembly can also import foreign functions (Javascript functions in the browser). You can get output by reading the buffer at the end of execution/when receiving callbacks. So, for instance, you pass a few frames worth of buffers to WASM, WASM renders pixels into the buffers, calls a callback, and the Javascript reads data from the buffer (sending it to a <canvas> or similar). The benefit of WASM is that it can't be very malicious by itself. It requires the runtime to provide it with exported functions and callbacks to do any file I/O, network I/O, or spawning new tasks. Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to, while WASM can only interact with the specific API surface you provide it. That makes WASM less powerful, but more predictable, and in my opinion better for building integrations with as there is no risk of internal APIs being accessed (that you will be blamed for if they break in an update).
- panic 2y ago
  I don’t believe it is currently possible for a WebAssembly instance to access any buffer other than its own memory. You have to copy data in and out.
  - deathanatos 2y ago
    The embedder could hand the module functions for manipulating external buffers via externrefs. (I'm not sure if that's a good idea, or not, just that it could.) But if the module wants to compute on the values in the buffer, at some level it would have to copy the data in/out.
  - davexunit 2y ago
    Use the GC instructions and you can freely share heap references amongst other modules and the host.
    - panic 2y ago
      How do you access the contents of a heap reference from JavaScript in order to “send it to a <canvas> or similar”?
      - davexunit 2y ago
        Assuming you're talking about reading binary data like (array i8), the GC MVP doesn't have a great answer right now. Have to call back into wasm to read the bytes. Something for the group to address in future proposals. Sharing between wasm modules is better right now.
- brabel 2y ago
  > Lua and similar tools can go deep into the runtime they exist in, altering system state and messing with system memory if they want to That's not correct, when you embed Lua you can choose which APIs are available, to make the full stdlib available you must explicitly call `luaL_openlibs` [1]. [1] https://www.lua.org/manual/5.3/manual.html#luaL_openlibs https://www.lua.org/manual/5.3/manual.html#luaL_openlibs
Karellen 2y ago
> You write in one language Not quite. Web assembly isn't a source language, it's a compiler target. So you should be able to write in C, Rust, Fortran, or Lua and compile any of those to WebAssembly. Except that WebAssembly is a cross-platform assembly language/machine code which is very similar to the native machine code of many/most contemporary CPUs. This means a WebAssembly interpreter can be very straightforward, and could often translate one WebAssembly instruction to one native CPU instruction. Or rather, it can compile a stream of WebAssembly instructions almost one-to-one to native CPU instructions, which it can then execute directly.
- whizzter 2y ago
  A JIT should be able to translate most arithmetic and binary instructions to single-opcodes, however anything involving memory and functions calls needs safety checks that becomes multi-instruction. branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops (if it also wants to be crash-safe even if it's exploit safe).
  - kouteiheika 2y ago
    > anything involving memory [..] needs safety checks that becomes multi-instruction Not necessarily; on AMD64 you can do memory accesses in a single instruction relatively easily by using the CPU's paging machinery for safety checks plus some clever use of address space. > branches could mostly be direct _unless_ the runtime has any kind of metering (it should) to stop eternal loops Even with metering the branches would be direct, you'd just insert the metering code at the start of each basic block (so that's two extra instructions at the start of each basic block). Or did you mean something else?
    - charleslmunger 2y ago
      Metering also doesn't require a branch if you implement it with page faults. See "Implicit suspend checks" in https://android-developers.googleblog.com/2023/11/the-secret-to-androids-improved-memory-latest-android-runtime-update.html?m=1 https://android-developers.googleblog.com/2023/11/the-secret...
pdubroy 2y ago
You should check out the book :-) We have a chapter called "What Makes WebAssembly Safe?" which covers the details. You can get a sneak peek here: https://bsky.app/profile/wasmgroundup.com/post/3lh2e4eiwnm2p https://bsky.app/profile/wasmgroundup.com/post/3lh2e4eiwnm2p
coliveira 2y ago
I think the biggest advantage of wasm in terms of security is that it doesn't accept machine language written in the target machine, only in this artificial machine language. This means that it cannot encode arbitrary code that could be executed by the host machine. Everything it runs has necessarily to go through the wasm interpreter.
- hb-robo 2y ago
  That's quite interesting. This is way outside of my wheelhouse - has this kind of approach been tried in other security contexts before? What would you even call that, virtualization?
  - tubs 2y ago
    Java.
  - dmitrygr 2y ago
    The word is "bytecode" and the idea is as old as computing.
- wyldfire 2y ago
  > This means that it cannot encode arbitrary code that could be executed by the host machine. But the host machine still can, so it's not as big of advantage in that regard. If you could somehow deliver a payload of native code and jump to it, it'd work just fine. But the security you get is the fact that it's really hard to do that because there's no wasm instructions to jump to arbitrary memory locations (even if all the host ISAs do have those). Having a VM alone doesn't provide security against attacks. It's often the case that VMs are used with memory-safe languages and those languages' runtime bounds checks and other features are what gives them safety moreso than their VM. In fact, most bytecode languages provide a JIT (including some wasm deployments) so you're actually running native code regardless.
Gupta2 2y ago
Speaking of WebAssembly security, is it vulnerable to Spectre/CPU style attacks like those in JavaScript? (WASM without imported JS functions)
- saagarjha 2y ago
  Yes.
- jeffparsons 2y ago
  Yes, if you give the Wasm instance access to timers.
pizlonator 2y ago
> But I've always gotten confused with... it is secure because by default it can't do much. Yes. That’s a super accurate description. You’re not confused. > I don't quite understand how to view WebAssembly. You write in one language, it compiles things like basic math (nothing with network or filesystem) to another and it runs in an interpreter. Almost. Wasm is cheap to JIT compile and the resulting code is usually super efficient. Sometimes parity with native execution. > I feel like I have a severe lack/misunderstanding. There's a ton of hype for years, lots of investment... but it isn't like any case where you want to add Lua to an app you can add WebAssembly/vice versa? It’s definitely a case where the investment:utility ratio is high. ;-) Here’s the trade off between embedding Lua and embedding Wasm: - Both have the problem that they are only as secure as the API you expose to the guest program. If you expose `rm -rf /` to either Lua or Wasm, you’ll have a bad time. And it’s surprisingly difficult to convince yourself that you didn’t accidentally do that. Security is hard. - Wasm is faster than Lua. - Lua is a language for humans, no need for another language and compiler. That makes Lua a more natural choice for embedded scripting. - Lua is object oriented, garbage collected, and has a very principled story for how that gets exposed to the host in a safe way. Wasm source languages are usually not GC’d. That means that if you want to expose object oriented API to the guest program, then it’ll feel more natural to do that with Lua. - The wasm security model is dead simple and doesn’t (necessarily) rely on anything like GC, making it easier to convince yourself that the wasm implementation is free of security vulnerabilities. If you want a sandboxed execution environment then Wasm is better for that reason.
jedisct1 2y ago
It's like JavaScript, Python or PHP. There are no pointers, only arrays, that cause the app to crash if it attempts to read out of the bounds.

veltas 2y ago

> actually quite readable, although some of the language can be a bit intimidating if you're not used to reading programming language papers You're more generous than me, I think it's rubbish. Would have been easier to read if they had written it more like an ISA manual.

amw-zero 2y ago
This is an opportunity to learn. The way WebAssembly is defined is the standard way PL semantics are defined.
- veltas 2y ago
  WebAssembly isn't a programming language, it's an ISA or a bytecode VM. They've defined it in a very over-engineered way, with different representations all the way through. It has created issues for people working on assemblers and code generation because it's hard to choose a good representation for compilers to generate. This all could have been avoided.
  - amw-zero 2y ago
    Those are all programming languages.
mananaysiempre 2y ago
You can understand the WASM spec in your sleep if you’ve ever worked through a type-system paper from the last two decades (or a logic paper from even earlier I guess). Granted, not many people have, but there’s a reason why it makes sense for it to be written in that style: they want it to be very clear that the verification (typechecking, really) algorithm doesn’t have any holes, and for that it’s reasonable to speak the language of the people who prove that type of thing for a living. The WASM spec is also the ultimate authoritative reference for both programmers and implementers. That’s different from the goals of an ISA manual, which usually only targets programmers and just says “don’t do that” for certain dark corners of the (sole) implementation. (The RISC-V manual is atypical in this respect; still, I challenge you to describe e.g. which PC value the handler will see if the user code traps on a base RV32IMA system.)

deleted 2y ago

[deleted]

amw-zero 2y ago

I think it's much better to just learn how to read inference rules. They're actually quite simple, and are used ubiquitously to define PL semantics definitions. Constraining this on "that's not an option" is a big waste of time - learning this will open up all of the literature written on the subject.

shpongled 2y ago
The WASM spec is so well defined presumably because Andreas Rossberg is the editor - and he did a bunch of PL research on extensions to Standard ML, which is famous for it's specification!
- amw-zero 2y ago
  I totally agree. Standard ML set the stage for everyone to finally formally specify a full language, and only WebAssembly has carried the torch as far as I know (other than small research languages).

crabmusket 2y ago

I bought the early access of your book a while ago and completed the first few chapters which were available. I found it a fantastic resource, and I was keen to continue but other responsibilities have gotten in the way since. I recommend it!

pdubroy 2y ago
Glad to hear! If you haven't looked at it in a while — we just published a draft of the final technical chapter, and are planning an official launch on March 4. So, might be a good time to dig back in :-)