How to Avoid Segmentation Faults When Using the Base Pointer Register (%rbp) in Inline Assembly?

Using the base pointer register (%rbp) within inline asm

Inline assembly (inline asm) is a technique that allows the inclusion of assembly language instructions directly within C code. Using inline asm with the base pointer register (%rbp) is a common requirement to perform various tasks. However, it is crucial to understand how to correctly use %rbp within inline asm to avoid issues.

In the provided code example:

void Foo(int &x)
{
    asm volatile ("pushq %%rbp;"         // 'prologue'
                  "movq %%rsp, %%rbp;"   // 'prologue'
                  "subq , %%rsp;"     // make room

                  "movl , -12(%%rbp);" // some asm instruction

                  "movq %%rbp, %%rsp;"  // 'epilogue'
                  "popq %%rbp;"         // 'epilogue'
                  : : : );
    x = 5;
}

The goal is to perform some assembly instructions while preserving the current stack frame by pushing and popping %rbp, the base pointer register. However, accessing the variable x after the inline asm results in a segmentation fault. This is because the inline asm modifies the stack frame in a way that corrupts the stored value of %rbp.

Understanding the Problem:

The error arises because the push instruction in the inline asm pushes a value onto the stack in the red zone below %rsp, where the compiler had stored an important value. The red zone is an area of memory reserved for use by the compiler and operating system during function calls. By pushing a value into this area, the inline asm clobbers the stored value, leading to the segmentation fault when trying to access x.

Solution:

To resolve this issue, avoid using the red zone for any stack manipulations within the inline asm. There are several ways to achieve this:

1. Use a memory operand for scratch space: Declare temporary variables in the non-inline asm code, such as an array, and pass their addresses to the inline asm as memory operands. Data can be written to and read from these variables within the inline asm.
2. Allocate/Deallocate stack space manually: If you need more stack space, manually allocate it before the inline asm using subq $12, %rsp and deallocate it after using addq $12, %rsp. However, be cautious not to corrupt nearby values on the stack.
3. Adjust stack pointer manually: Leave additional space on the stack before entering the inline asm with subq $128, %rsp and before exiting with addq $128, %rsp. This allows for using the full stack space without worrying about the red zone.

General Guidelines for Using Inline Asm:

• Use inline asm minimally, only when necessary for operations that cannot be efficiently implemented in C .
• Follow proper syntax and semantics for assembly instructions and constraints.
• Avoid clobbering registers or memory areas that may interfere with the surrounding code.
• Communicate with the compiler using correct input and output constraints to ensure proper function calling conventions and data handling.

The above is the detailed content of How to Avoid Segmentation Faults When Using the Base Pointer Register (%rbp) in Inline Assembly?. For more information, please follow other related articles on the PHP Chinese website!