It's fragile to try to retrieve the stack pointer by taking the address
of a variable on the stack. For instance, clang reserves more stack
space than gcc here, indicating that the variable may not be at the
start of the stack. Instead of relying upon this to work, retrieve the
"%rbp" value, which contains the value of "%rsp" before stack
allocation.
Signed-off-by: Bill Wendling <morbo@xxxxxxxxxx>
---
x86/realmode.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/x86/realmode.c b/x86/realmode.c
index cf45fd6..7c89dd1 100644
--- a/x86/realmode.c
+++ b/x86/realmode.c
@@ -518,11 +518,12 @@ extern void retf_imm(void);
static void test_call(void)
{
- u32 esp[16];
u32 addr;
inregs = (struct regs){ 0 };
- inregs.esp = (u32)esp;
+
+ // At this point the original stack pointer is in %ebp.
+ asm volatile ("mov %%ebp, %0" : "=rm"(inregs.esp));
MK_INSN(call1, "mov $test_function, %eax \n\t"
"call *%eax\n\t");
--
2.23.0.700.g56cf767bdb-goog
