[LLVMdev] Building a stable bitcode format for PNaCl - based on LLVM IR

[Mark Seaborn]

On 19 June 2013 12:42, Dmitri Rubinstein
<dmitri.rubinstein at googlemail.com>wrote:

> Am 19.06.2013 18:01, schrieb JF Bastien:
>
>>      From the provided documentation I understood that in memory data
>>     structures of a PNaCl program are incompatible to the host program
>>     because ABIs are different (e.g. PNaCl pointers are always 32-bit
>>     even when running on x86_64 platform).
>>     So PNaCl program can't access any data structures of the host
>>     program directly. The only communication way is by using syscalls,
>>     but the document does not specify syscalls in detail.
>>
>>
>> We should probably clarify in the final documentation, but the goal of
>> PNaCl is to not only be portable and fast but also be safe for the user,
>> and the way this is achieved is through NaCl's SFI. Specifically for
>> syscalls:
>> http://www.chromium.org/**nativeclient/reference/**anatomy-of-a-sys<http://www.chromium.org/nativeclient/reference/anatomy-of-a-sys>
>> In a way the syscalls offered are defined by the embedding sandbox: NaCl
>> through Chrome and sel_ldr have documented interfaces, and the NaCl SDK
>> offers POSIX-like interfaces built on top of these.
>>
>
> But this discussion is about stable bitcode format, or do you want to
> restrict the set of syscalls on the LLVM level ?
> I am interested for my project in having a stable portable bitcode format
> like the one you propose, but not as a part of a browser and I possibly
> need to have an extended set of syscalls. The documentation does not
> provide an example how the syscall is represented in the LLVM bitcode. Is
> this just a function call ?
>

Yes, any calls to interact with the world outside the sandbox are just done
via function calls.  The PNaCl program defines an entry point, _start(),
which gets passed a data structure containing a pointer to an interface
query function.  The user code can call this query function to get function
pointers for further interfaces such as write(), mmap(), thread_create(),
and others.  These interfaces are defined in
https://src.chromium.org/viewvc/native_client/trunk/src/native_client/src/untrusted/irt/irt.h?revision=11525.
This interface layer is already used by NaCl, so it's somewhat orthogonal
to PNaCl's subset of LLVM IR.

Cheers,
Mark
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