Re: [edk2-devel] Casting i128 into f64 in UEFI Rust pagefaults

["Andrew Fish" <afish@apple.com>]

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> On Jul 25, 2022, at 10:43 PM, Ayush Singh <ayushdevel1325@gmail.com> wrote:
> 
> Hi Andrew. Thanks for all your work. The more I look at this, the more it feels like it might be a problem on the LLVM side instead of Rust. I also found some more tests (all related to numbers btw) which can cause different types of exceptions, so I think I will try filing bugs upstream. 

Ayush,

In general If we want to move to Rust we are going to need a way to debug issues like this down to the root cause. I think figuring out how to debug will make it easier to move forward with the Rust port in general. It will time well spent. 

The best way to get LLVM fixed, if it is even broken, is to provide a simple test case that reproduces the behavior. I don’t think at this point we know what is going on. It is very unlikely that some random LLVM developer is going to invest the time in trying to setup some UEFI environment to try and root cause this bug. I general find I have to create a simple at desk example and then I get stuff fixed quickly. Basically a test case the LLVM developer can compile at their desk and see the error in assembler, or at least run it at desk and have bogus output. 

I’m not 100% sure what toolchain you are using. Can you `objdump -d hello_world_std.efi`and get some symbols with the disassembly? For VC++ I think it would be `DUMPBIN /DISASM`.

What are you planning on using for source level debugging Rust? I wrote some gdb[1] and lldb[2] debugging commands in Python. I’m guessing loading Rust symbols from PE/COFF images should be similar, as long as the debugger knows about rust. 

I’m happy to help you figure out stuff related to debugging Rust. 

[1] https://github.com/tianocore/edk2/blob/master/BaseTools/Scripts/efi_gdb.py
[2] https://github.com/tianocore/edk2/blob/master/BaseTools/Scripts/efi_lldb.py

Thanks,

Andrew Fish

> Yours Sincerely,
> 
> Ayush Singh
> 
> 
> 
> On 7/26/22 00:24, Andrew Fish wrote:
>> I guess I could at least dump to the end (req)…. Going backwards is a bit painful in x86. 
>> 
>> (lldb) dis -s 0x0000000140001B60 -b -c 30
>> hello_world_std.efi[0x140001b60]: 48 8b 09                       movq   (%rcx), %rcx
>> hello_world_std.efi[0x140001b63]: 48 01 c1                       addq   %rax, %rcx
>> hello_world_std.efi[0x140001b66]: 4c 89 c2                       movq   %r8, %rdx
>> hello_world_std.efi[0x140001b69]: 48 11 c2                       adcq   %rax, %rdx
>> hello_world_std.efi[0x140001b6c]: 48 31 c1                       xorq   %rax, %rcx
>> hello_world_std.efi[0x140001b6f]: 48 31 c2                       xorq   %rax, %rdx
>> hello_world_std.efi[0x140001b72]: 48 be 00 00 00 00 00 00 00 80  movabsq $-0x8000000000000000, %rsi ; imm = 0x8000000000000000 
>> hello_world_std.efi[0x140001b7c]: 4c 21 c6                       andq   %r8, %rsi
>> hello_world_std.efi[0x140001b7f]: e8 5c 55 00 00                 callq  0x1400070e0
>> hello_world_std.efi[0x140001b84]: 48 09 f0                       orq    %rsi, %rax
>> hello_world_std.efi[0x140001b87]: 48 83 c4 20                    addq   $0x20, %rsp
>> hello_world_std.efi[0x140001b8b]: 5e                             popq   %rsi
>> hello_world_std.efi[0x140001b8c]: c3                             retq   
>> hello_world_std.efi[0x140001b8d]: cc                             int3   
>> hello_world_std.efi[0x140001b8e]: cc                             int3   
>> hello_world_std.efi[0x140001b8f]: cc                             int3   
>> hello_world_std.efi[0x140001b90]: e9 db 55 00 00                 jmp    0x140007170
>> hello_world_std.efi[0x140001b95]: cc                             int3   
>> …
>> 
>> Then we can guess based on how functions get aligned to find the start….
>> 
>> hello_world_std.efi[0x140001b50]: 56                                   pushq  %rsi
>> hello_world_std.efi[0x140001b51]: 48 83 ec 20                          subq   $0x20, %rsp
>> hello_world_std.efi[0x140001b55]: 4c 8b 41 08                          movq   0x8(%rcx), %r8
>> hello_world_std.efi[0x140001b59]: 4c 89 c0                             movq   %r8, %rax
>> hello_world_std.efi[0x140001b5c]: 48 c1 f8 3f                          sarq   $0x3f, %rax
>> hello_world_std.efi[0x140001b60]: 48 8b 09                             movq   (%rcx), %rcx
>> hello_world_std.efi[0x140001b63]: 48 01 c1                             addq   %rax, %rcx
>> hello_world_std.efi[0x140001b66]: 4c 89 c2                             movq   %r8, %rdx
>> hello_world_std.efi[0x140001b69]: 48 11 c2                             adcq   %rax, %rdx
>> hello_world_std.efi[0x140001b6c]: 48 31 c1                             xorq   %rax, %rcx
>> hello_world_std.efi[0x140001b6f]: 48 31 c2                             xorq   %rax, %rdx
>> hello_world_std.efi[0x140001b72]: 48 be 00 00 00 00 00 00 00 80        movabsq $-0x8000000000000000, %rsi ; imm = 0x8000000000000000 
>> hello_world_std.efi[0x140001b7c]: 4c 21 c6                             andq   %r8, %rsi
>> hello_world_std.efi[0x140001b7f]: e8 5c 55 00 00                       callq  0x1400070e0
>> hello_world_std.efi[0x140001b84]: 48 09 f0                             orq    %rsi, %rax
>> hello_world_std.efi[0x140001b87]: 48 83 c4 20                          addq   $0x20, %rsp
>> hello_world_std.efi[0x140001b8b]: 5e                                   popq   %rsi
>> hello_world_std.efi[0x140001b8c]: c3                                   retq   
>> 
>> So the faulting function is getting passed a bad pointer as its 1st arg. 
>> 
>> Thanks,
>> 
>> Andrew Fish
>> 
>>> On Jul 25, 2022, at 11:45 AM, Andrew Fish <afish@apple.com> <mailto:afish@apple.com> wrote:
>>> 
>>> Ops… Looks like your PE/COFF is linked at 0x0000000140000000, so 0x140001b60 is the interesting bit.
>>> 
>>> (lldb) dis -s 0x0000000140001B60 -b
>>> hello_world_std.efi[0x140001b60]: 48 8b 09                       movq   (%rcx), %rcx
>>> hello_world_std.efi[0x140001b63]: 48 01 c1                       addq   %rax, %rcx
>>> hello_world_std.efi[0x140001b66]: 4c 89 c2                       movq   %r8, %rdx
>>> hello_world_std.efi[0x140001b69]: 48 11 c2                       adcq   %rax, %rdx
>>> hello_world_std.efi[0x140001b6c]: 48 31 c1                       xorq   %rax, %rcx
>>> hello_world_std.efi[0x140001b6f]: 48 31 c2                       xorq   %rax, %rdx
>>> hello_world_std.efi[0x140001b72]: 48 be 00 00 00 00 00 00 00 80  movabsq $-0x8000000000000000, %rsi ; imm = 0x8000000000000000 
>>> hello_world_std.efi[0x140001b7c]: 4c 21 c6                       andq   %r8, %rsi
>>> 
>>>  RCX - FFFFFFFFFFFFFFFF
>>> 
>>> So yea that looks like the fault. 
>>> 
>>> I don’t see that pattern in your .s file…. 
>>> 
>>> Can you figure out what function is @ 0x140001b60 in the PE/COFF image. Do you have a map file from the linker?
>>> 
>>> Thanks,
>>> 
>>> Andrew Fish
>>> 
>>> PS Again sorry I don’t have anything installed to crack PDB files. 
>>> 
>>> Thanks,
>>> 
>>> Andrew Fish
>>> 
>>>> On Jul 25, 2022, at 10:51 AM, Andrew Fish via groups.io <afish=apple.com@groups.io> <mailto:afish=apple.com@groups.io> wrote:
>>>> 
>>>> Ayush,
>>>> 
>>>> CR2 is the fault address so 0xFFFFFFFFFFFFFFFF. Given for EFI Virt == Physical the fault address looks like a bad pointer. 
>>>> 
>>>> Sorry I’ve not used VC++ in a long time so I don’t know how to debug with VC++, but If I was using clang/lldb I’d look at the source and assembly for the fault address. 
>>>> 
>>>> The image base is: 0x000000000603C000
>>>> The fault PC/RIP is: 000000000603DB60
>>>> 
>>>> So the faulting code is at 0x1B60 in the image. Given the images are linked at zero you should be able to load the build product into the debugger and look at what code is at offset 0x1B60. The same should work for any tools that dump the binary. 
>>>> 
>>>> Thanks,
>>>> 
>>>> Andrew Fish
>>>> 
>>>>> On Jul 25, 2022, at 10:33 AM, Ayush Singh <ayushdevel1325@gmail.com> <mailto:ayushdevel1325@gmail.com> wrote:
>>>>> 
>>>>> Hello everyone.While running Rust tests in UEFI environment, I have come across a numeric test that causes a pagefault. A simple reproducible example for this is given below:
>>>>> 
>>>>> ```rust
>>>>> 
>>>>> fn main() {
>>>>>     use std::hint::black_box as b;
>>>>> 
>>>>>     let z: i128 = b(1);
>>>>>     assert!((-z as f64) < 0.0);
>>>>> }
>>>>> 
>>>>> ```
>>>>> 
>>>>> 
>>>>> The exception output is as follows:
>>>>> 
>>>>> ```
>>>>> 
>>>>> !!!! X64 Exception Type - 0E(#PF - Page-Fault)  CPU Apic ID - 00000000 !!!!
>>>>> ExceptionData - 0000000000000000  I:0 <i:0> R:0 <r:0> U:0 <u:0> W:0 <w:0> P:0 <p:0> PK:0 <pk:0> SS:0 <ss:0> SGX:0 <sgx:0>
>>>>> RIP  - 000000000603DB60, CS  - 0000000000000038, RFLAGS - 0000000000000246
>>>>> RAX  - 0000000000000000, RCX - FFFFFFFFFFFFFFFF, RDX - FFFFFFFFFFFFFFFF
>>>>> RBX  - 0000000000000000, RSP - 0000000007EDF1D0, RBP - 0000000007EDF4C0
>>>>> RSI  - 0000000007EDF360, RDI - 0000000007EDF3C0
>>>>> R8   - 0000000000000000, R9  - 0000000000000038, R10 - 0000000000000000
>>>>> R11  - 0000000000000000, R12 - 00000000060C6018, R13 - 0000000007EDF520
>>>>> R14  - 0000000007EDF6A8, R15 - 0000000005FA9490
>>>>> DS   - 0000000000000030, ES  - 0000000000000030, FS  - 0000000000000030
>>>>> GS   - 0000000000000030, SS  - 0000000000000030
>>>>> CR0  - 0000000080010033, CR2 - FFFFFFFFFFFFFFFF, CR3 - 0000000007C01000
>>>>> CR4  - 0000000000000668, CR8 - 0000000000000000
>>>>> DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
>>>>> DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
>>>>> GDTR - 00000000079DE000 0000000000000047, LDTR - 0000000000000000
>>>>> IDTR - 0000000007418018 0000000000000FFF,   TR - 0000000000000000
>>>>> FXSAVE_STATE - 0000000007EDEE30
>>>>> !!!! Find image based on IP(0x603DB60) /var/home/ayush/Documents/Programming/Rust/uefi/hello_world_std/target/x86_64-unknown-uefi/debug/deps/hello_world_std-338028f9369e2d42.pdb (ImageBase=000000000603C000, EntryPoint=000000000603D8C0) !!!!
>>>>> 
>>>>> ```
>>>>> 
>>>>> 
>>>>> From my testing, the exception only occurs when a few conditions are met.
>>>>> 
>>>>> 1. The binary is compiled in Debug mode. No error in Release mode.
>>>>> 
>>>>> 2. `i128` is in a black_box [1]. Does not occur if `black_box` is not present.
>>>>> 
>>>>> 3. It has to be `i128`. `i64` or something else work fine.
>>>>> 
>>>>> 4. The cast has to be done on `-z`. Doing the same with `+z` is fine.
>>>>> 
>>>>> 
>>>>> I have also been discussing this in the Rust zulipchat [2], so feel free to chime in there.
>>>>> 
>>>>> 
>>>>> Additionally, here are links for more information about this program:
>>>>> 
>>>>> 1. Assembly: https://rust-lang.zulipchat.com/user_uploads/4715/od51Y9Dkfjahcg9HHcOud8Fm/hello_world_std-338028f9369e2d42.s
>>>>> 
>>>>> 2. EFI Binary: https://rust-lang.zulipchat.com/user_uploads/4715/CknqtXLR8SaJZmyOnXctQkpL/hello_world_std.efi
>>>>> 
>>>>> 3. PDB file: https://rust-lang.zulipchat.com/user_uploads/4715/zV4i6DsjgQXotp_gS1naEsU0/hello_world_std-338028f9369e2d42.pdb
>>>>> 
>>>>> 
>>>>> Yours Sincerely,
>>>>> 
>>>>> Ayush Singh
>>>>> 
>>>>> 
>>>>> [1]: https://doc.rust-lang.org/std/hint/fn.black_box.html
>>>>> 
>>>>> [2]: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Casting.20i128.20to.20f64.20in.20black_box.20causes.20exception.20in.20UEFI
>>>>> 
>>>>> 
>>>>> 
>>>> 
>>>> 
>>> 
>> 


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