GDB qemu "Cannot access memory at address..."

Question

I have a simple program that I am using to test a python riscv disassembler I am making and I want to use gdb/qemu to test my work. The program is this literally just this:

int main(int argc, char *argv[]) {
    while (1);
    return 0;
}

this is the command I am using to start gdb: gdb-multiarch ./test -ex "target remote :7224" -ex "tbreak main:4" -ex "continue"

This is what was used to compile it: riscv64-linux-gnu-gcc -o test test.c

But I am getting this error when I try to change any memory values:

(gdb) disassemble
Dump of assembler code for function main:
   0x00000040000005ea <+0>:     addi    sp,sp,-32
=> 0x00000040000005ec <+2>:     sd      s0,24(sp)
   0x00000040000005ee <+4>:     addi    s0,sp,32
   0x00000040000005f0 <+6>:     mv      a5,a0
   0x00000040000005f2 <+8>:     sd      a1,-32(s0)
   0x00000040000005f6 <+12>:    sw      a5,-20(s0)
   0x00000040000005fa <+16>:    j       0x40000005fa <main+16>
End of assembler dump.
(gdb) set *(int*) $pc = 0x2e325f43
Cannot access memory at address 0x40000005ec

I just want to see what instruction gdb interprets with the bytes I set. Google has been little to no help with this. What could I be doing wrong?

Answer 1

Figured it out in a stupid manner.

set $pc = $sp

Then I can change the pc

Answer 2

This command:

set *(int*) $pc = 0x2e325f43

is trying to write a value to the memory the PC currently points at (that's 0x00000040000005ec in this case). As it happens, that memory is read-only, which is pretty usual for areas of memory with code in them[*]. So gdb tells you it can't write there. You should be able to write to memory which isn't read-only.

[*] With a suitable linker map you can create binaries which have the code in writeable memory. But the default for Linux executables is that code segments are read-only.

Your other command:

set $pc = $sp

changes the PC; it sets it to whatever the stack pointer is pointing at. That's going to be fatal for any further attempts to execute code, unless you put some code there, of course. As it happens, the stack is generally writeable, which is why writing to the memory pointed to by the PC then works.