Why Does Using the Base Pointer (%rbp) in C Inline Assembly Cause Segfaults?

Issue: Using the Base Pointer Register (%rbp) in C Inline Assembly

When using inline assembly to access the base pointer register (%rbp) in a C function, an unexpected seg fault occurs after accessing the function's argument.

Explanation

The problem lies in stepping on the "red zone" below RSP (or subtract pointer), which stores values that GCC needs to keep track of. In the example code provided, a push instruction is used within the inline assembly, which decrements %rsp by 8 and overwrites the lower 32 bits of the function's argument (&x).

When the inline assembly is complete, GCC attempts to use the clobbered value as an address for a 4-byte store, leading to the seg fault.

Solution

There are three main solutions to this issue:

1. Use a "memory" output operand (such as a scratch array) for workspace and do not read from it.
2. Skip over the red zone by using add $-128, %rsp / sub $-128, %rsp around the assembly code.
3. Compile with -mno-red-zone (but note that this is not a per-function option).

Alternative Approach

Instead of using a red zone overrun, consider explicitly allocating workspace within the inline assembly using an "=m" constraint, as seen in the following corrected code example:

void Bar(int &x)
{
    int tmp;
    long tmplong;
    asm ("lea  -16 + %[mem1], %%rbp\n\t"
         "imul , %%rbp, %q[reg1]\n\t"  // q modifier: 64bit name.
         "add  %k[reg1], %k[reg1]\n\t"    // k modifier: 32bit name
         "movl , %[mem1]\n\t" // some asm instruction writing to mem
           : [mem1] "=m" (tmp), [reg1] "=r" (tmplong)
           :
           : "%rbp"
         );
    x = 5;
}

Here, the "=m" constraint ensures that workspace is allocated within the red zone and avoids overwriting the function argument.

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