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

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

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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> 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 R:0 U:0 W:0 P:0 PK:0 SS: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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