Prxgen Static Alignment Issue Unloadable PRX Generation Above 32 Bytes

July 5, 2025 by StackCamp Team 71 views

Prxgen Static Alignment Issue Unloadable PRX Generation with Alignments Greater Than 32 Bytes

Discussion category : overdrivenpotato, rust-psp

Introduction

In this article, we delve into a perplexing issue encountered during the generation of PRX (PlayStation Portable executable) files, specifically concerning static alignment. The problem arises when attempting to use alignments greater than 32 bytes, leading to unloadable PRX files and runtime errors. This issue, discovered during the development of a Rust-based PSP application, highlights the intricacies of memory alignment and its impact on executable loading and execution within the PSP environment. This article aims to explore the technical details of this static alignment issue, the error messages, and the debugging process that led to the identification of the root cause. We'll examine the code snippets that trigger the problem, the error logs generated by the PSP, and the analysis of the memory addresses involved. Furthermore, we'll discuss potential solutions and workarounds, including a temporary patch that has been implemented to mitigate the issue. By understanding the intricacies of this problem, developers can avoid similar pitfalls and contribute to more robust and stable PSP applications.

Code Snippet

The issue was initially identified in a Rust project targeting the PSP. The following code snippet demonstrates the problem:

#![no_std]
#![no_main]

use psp;

psp::module!("minrepro", 1, 1);

#[repr(align(32))]
struct Align32<T>(T);

#[repr(align(64))]
struct Align64<T>(T);

#[repr(align(128))]
struct Align128<T>(T);

static MESSAGE1: Align32<&str> = Align32("hello world"); // this is fine
static MESSAGE2: Align64<&str> = Align64("hello world"); // this is *NOT* fine
static MESSAGE3: Align128<&str> = Align128("hello world"); // this is *ALSO NOT* fine

fn psp_main() {
    psp::dprintln!("{}", MESSAGE2.0);
}

In this code, the #[repr(align(n))] attribute is used to specify the alignment of the structs Align32, Align64, and Align128. The MESSAGE1 static variable, aligned to 32 bytes, works correctly. However, MESSAGE2 and MESSAGE3, aligned to 64 and 128 bytes respectively, cause the PRX file to fail to load, resulting in a bus error during runtime. This discrepancy points to a limitation or bug in the PRX generation or loading process when dealing with alignments greater than 32 bytes.

Alignment in Memory

Memory alignment is a critical concept in computer architecture, ensuring that data is stored at addresses that are multiples of a certain value. This is often dictated by the hardware, particularly the CPU, which may have performance penalties for accessing data at unaligned addresses. For instance, a 4-byte integer might need to be stored at an address that is a multiple of 4. In the context of the PSP, which uses a MIPS architecture processor, alignment is similarly crucial. The #[repr(align(n))] attribute in Rust allows developers to specify the alignment requirements for data structures, ensuring they are placed in memory at appropriate boundaries. However, as the code snippet demonstrates, simply specifying the alignment doesn't guarantee it will work correctly, especially when dealing with static data in PRX modules.

Static Variables and PRX Modules

Static variables, like MESSAGE2 and MESSAGE3, are allocated in a specific memory section of the executable, typically the .rodata section for read-only data. When a PRX module is loaded into memory, the PSP's loader must correctly map these sections and ensure that the data adheres to the specified alignment. The issue here suggests that the loader or the PRX generation process isn't handling alignments greater than 32 bytes correctly for static variables. This could be due to a bug in the loader, an incorrect calculation of memory offsets, or a misalignment between the linker's assumptions and the actual memory layout enforced by the PSP's kernel.

Error Log Analysis

When the problematic PRX file is executed on the PSP, a bus error occurs. The following error log provides crucial information for diagnosing the issue:

host0:/> ./target/mipsel-sony-psp/release/min-repro.prx
Exception - Bus error (data)
Thread ID - 0x00CB8A29
Th Name   - SceKernelModmgrWorker
Module ID - 0x002B6149
Mod Name  - sceLoaderCore
EPC       - 0x8801E2FC
Cause     - 0x1000001C
BadVAddr  - 0x679B6CB1
Status    - 0x20088603
Address   - 0x000072FC
zr:0x00000000 at:0x00007F01 v0:0x80020000 v1:0x00000001
a0:0x00008378 a1:0x882FAAF0 a2:0x0000006D a3:0x00000000
t0:0x00000000 t1:0x88016A18 t2:0x0000006D t3:0x00008388
t4:0x00032EF2 t5:0x0000010E t6:0x00000001 t7:0x08810E00
s0:0x00000000 s1:0x8821313C s2:0x00000000 s3:0x0880D8B4
s4:0x00000000 s5:0x0880D8B0 s6:0x00000002 s7:0x882FAD90
t8:0x0000CE00 t9:0x00034D00 k0:0x00000000 k1:0x00000000
gp:0x8806BD90 sp:0x882FAAE0 fp:0x00000001 ra:0x8801D9B4
0x8801E2FC: 0x84830006 '....' - lh         $v1, 6($a0)

The error log indicates a bus error, specifically a data bus error, which occurs when the CPU attempts to access memory at an invalid address. The BadVAddr field, 0x679B6CB1, represents the virtual address that caused the error, while the EPC (Exception Program Counter) field, 0x8801E2FC, points to the instruction that triggered the exception. The instruction lh $v1, 6($a0) at address 0x8801E2FC is attempting to load a halfword (2 bytes) from the address $a0 + 6. The value of $a0 is 0x00008378, which, when added to 6, results in 0x0000837E. This address is likely unaligned for the data being accessed, leading to the bus error.

Tracing the Error

The error log also provides the module name sceLoaderCore, indicating that the error originates from the PSP's module loading core. This suggests that the issue lies in how the PRX module is being loaded and initialized. The error occurs within the SceKernelModmgrWorker thread, which is responsible for managing module loading and unloading. By tracing the execution flow back to the sceLoaderCore module, it becomes evident that the problem is related to how the module's data sections, particularly the .rodata section containing the static variables, are being handled.

Identifying the Root Cause

The analysis of the error log, combined with the code snippet, points to a misalignment issue within the .rodata section. The value of $a0, 0x00008378, is identified as the address of .lib.ent, which is related to library entries. However, the code expects $a0 to point to .rodata.sceKernelModuleInfo, which contains metadata about the module. This discrepancy suggests that the memory layout is not being correctly established during module loading. The fact that alignments greater than 32 bytes trigger the issue indicates that the loader might be making assumptions about the maximum alignment requirements, leading to incorrect address calculations.

In-depth Analysis and Debugging

To further investigate the root cause, a deeper dive into the PSP's module loading process and memory layout is necessary. The error log provides valuable clues, such as the invalid memory address and the instruction that triggered the exception. By examining the disassembly of the sceLoaderCore module and tracing the execution flow, it's possible to pinpoint the exact location where the misalignment occurs. The provided link to the uofw repository (https://github.com/uofw/uofw/blob/7ca6ba13966a38667fa7c5c30a428ccd248186cf/src/kd/loadcore/module.c#L563) leads to a specific line in the module.c file, which is part of the uofw project, an open-source PSP kernel. This line likely corresponds to the instruction that is attempting to access the misaligned data.

Examining the Memory Map

The image provided in the original report (https://github.com/user-attachments/assets/40df798c-54e7-4720-9929-b4f85003b961) is crucial for understanding the memory layout of the PRX module. It shows the different sections of the PRX file, such as .text (code), .rodata (read-only data), .data (initialized data), and .bss (uninitialized data). The .rodata section is of particular interest, as it contains the static variables that are causing the issue. By comparing the expected memory layout with the actual layout generated by the toolchain, it's possible to identify any discrepancies in alignment or section placement. The fact that the address 0x8378 points to .lib.ent instead of .rodata.sceKernelModuleInfo strongly suggests that the section headers or relocation information are not being processed correctly, leading to the misalignment.

Relocations and Section Headers

Relocations are instructions within the executable that tell the loader how to adjust memory addresses at runtime. When a PRX module is loaded, the loader uses the relocation information to update pointers and addresses to their correct locations in memory. Section headers, on the other hand, describe the different sections of the executable, including their size, alignment, and memory location. If the relocation information or section headers are not correctly generated or processed, it can lead to memory misalignment issues. In this case, it's possible that the alignment requirements specified by the #[repr(align(n))] attribute are not being properly encoded in the section headers or are not being honored by the loader during relocation.

The Role of the Toolchain

The toolchain, which includes the compiler, linker, and other utilities, plays a critical role in generating the PRX file. The compiler is responsible for translating the Rust code into MIPS assembly, while the linker combines the compiled code with libraries and generates the final executable. The linker is also responsible for laying out the different sections of the executable in memory and generating the section headers and relocation information. If there is a bug or misconfiguration in the toolchain, it can lead to incorrect alignment or section placement. For example, the linker might not be correctly handling alignment requests greater than 32 bytes, or it might be generating incorrect relocation entries for static variables in the .rodata section.

Workaround and Potential Solutions

To mitigate the issue, a temporary workaround has been implemented to patch out the alignment. This involves modifying the generated PRX file to remove the alignment requirements for static variables. While this workaround allows the PRX module to load and execute, it's not a permanent solution, as it could potentially lead to performance issues or other unexpected behavior. A more robust solution would involve fixing the root cause of the issue, either in the toolchain or in the PSP's module loader.

Investigating the Toolchain

One potential solution is to investigate the toolchain and identify any bugs or misconfigurations that might be causing the alignment issue. This could involve examining the linker scripts, compiler flags, and other settings to ensure that they are correctly configured to handle alignments greater than 32 bytes. It might also be necessary to modify the toolchain to correctly encode alignment requirements in the section headers and relocation information.

Patching the Module Loader

Another potential solution is to patch the PSP's module loader to correctly handle alignments greater than 32 bytes. This would involve modifying the sceLoaderCore module to ensure that it correctly calculates memory offsets and applies relocations for static variables in the .rodata section. However, patching the module loader is a more complex and risky approach, as it could potentially introduce instability or compatibility issues.

Long-Term Solution

The long-term solution would likely involve a combination of both approaches. The toolchain should be updated to correctly generate PRX files with alignments greater than 32 bytes, and the module loader should be updated to correctly handle these files. This would ensure that the PSP can load and execute PRX modules with any alignment requirements, without encountering bus errors or other issues. This fix will improve the Rust PSP toolchain and allow for more efficient memory usage and potentially improve performance by ensuring data is correctly aligned for the MIPS architecture.

Conclusion

The static alignment issue encountered during PRX generation highlights the importance of understanding memory alignment and its impact on executable loading and execution. The issue, triggered by alignments greater than 32 bytes, results in bus errors and prevents the PRX module from loading correctly. Through careful analysis of error logs, memory maps, and the PSP's module loading process, the root cause was traced to a misalignment in the .rodata section, likely due to incorrect section header or relocation processing. While a temporary workaround has been implemented, a more robust solution would involve fixing the toolchain and/or the PSP's module loader. This issue underscores the complexities of developing for embedded platforms like the PSP and the importance of thorough debugging and analysis to identify and resolve such problems. By addressing this issue, developers can create more stable and efficient PSP applications, pushing the boundaries of what's possible on this classic handheld console.