From: Pierre-Marc Fournier Date: Tue, 19 Jan 2010 20:23:04 +0000 (-0500) Subject: save_registers: add comments and make safer X-Git-Tag: v1.9.1~771 X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=55c5b393e9b197033ed1a6a8cc8f179412c61bf6;p=lttng-ust.git save_registers: add comments and make safer Carefully save all registers that could be changed by the function calls. --- diff --git a/include/ust/processor.h b/include/ust/processor.h index 4bd0997d..ba236124 100644 --- a/include/ust/processor.h +++ b/include/ust/processor.h @@ -25,6 +25,27 @@ struct registers { #ifdef CONFIG_UST_GDB_INTEGRATION +/* save_registers - saves most of the processor's registers so + * they are available to the probe. gdb uses this to give the + * value of local variables. + * + * Saving all registers without losing any of their values is + * tricky. + * + * We cannot pass to the asm stub the address of a registers structure + * on the stack, because it will use a register and override its value. + * + * We don't want to use a stub to push the regs on the stack and then + * another stub to copy them to a structure because changing %sp in asm + * and then returning to C (even briefly) can have unexpected results. + * Also, gcc might modify %sp between the stubs in reaction to the + * register needs of the second stub that needs to know where to copy + * the register values. + * + * So the chosen approach is to use another stack, declared in thread- + * local storage, to push the registers. They are subsequently copied + * to the stack, by C code. + */ #define save_registers(regsptr) \ asm volatile ( \ @@ -36,35 +57,47 @@ struct registers { "pushl %%eax\n\t" \ /* ebx is used for TLS access */ \ "pushl %%ebx\n\t" \ - /* ecx will be used to temporarily hold the stack bottom addr */ \ - "pushl %%ecx\n\t" \ - /* rdi is the input to __tls_get_addr, and also a temp var */ \ - "pushl %%edi\n\t" \ + /* ecx will be used to temporarily hold the stack bottom addr */\ + "pushl %%ecx\n\t" \ + /* rdi is the input to __tls_get_addr, and also a temp var */ \ + "pushl %%edi\n\t" \ + /* For TLS access, we have to do function calls. However, \ + * we must not lose the original value of: \ + * esp, eflags, eax, ebx, ecx, edx, esi, edi, ebp, cs, ss \ + * \ + * Some registers' original values have already been saved: \ + * esp, eflags, eax, ebx, ecx, edi \ + * \ + * In addition, the i386 ABI says the following registers belong\ + * to the caller function: \ + * esp, ebp, esi, edi, ebx \ + * \ + * The following registers should not be changed by the callee: \ + * cs, ss \ + * \ + * Therefore, the following registers must be explicitly \ + * preserved: \ + * edx \ + */ \ + "pushl %%edx\n\t" \ /* Get GOT address */ \ "call __i686.get_pc_thunk.bx\n\t" \ "addl $_GLOBAL_OFFSET_TABLE_, %%ebx\n\t" \ - /* Save registers before call (not using ecx yet but we must preserve \ - the original value of edx. */ \ - "pushl %%edx\n\t" \ /* Start TLS access of private reg stack pointer */ \ "leal ust_reg_stack_ptr@tlsgd(,%%ebx,1),%%eax\n\t" \ "call ___tls_get_addr@plt\n\t" \ /* --- End TLS access */ \ - "popl %%edx\n\t" \ /* check if ust_reg_stack_ptr has been initialized */ \ "movl (%%eax),%%ecx\n\t" \ "testl %%ecx,%%ecx\n\t" \ "jne 1f\n\t" \ "movl %%eax,%%ecx\n\t" \ - /* Save registers before call (using ecx and we must preserve \ - the original value of edx. */ \ + /* Save ecx because we are using it. */ \ "pushl %%ecx\n\t" \ - "pushl %%edx\n\t" \ /* Start TLS access of private reg stack */ \ "leal ust_reg_stack@tlsgd(,%%ebx,1),%%eax\n\t" \ "call ___tls_get_addr@plt\n\t" \ /* --- End TLS access */ \ - "popl %%edx\n\t" \ "popl %%ecx\n\t" \ "addl $500,%%eax\n\t" \ "movl %%eax,(%%ecx)\n\t" \ @@ -72,6 +105,8 @@ struct registers { /* now the pointer to the private stack is in eax. \ must add stack size so the ptr points to the stack bottom. */ \ "1:\n\t" \ + /* edx was pushed for function calls */ \ + "popl %%edx\n\t" \ /* Manually push esp to private stack */ \ "addl $-4,(%%eax)\n\t" \ "movl 20(%%esp), %%edi\n\t" \ @@ -187,6 +222,32 @@ struct registers { "pushq %%rbx\n\t" \ /* rdi is the input to __tls_get_addr, and also a temp var */ \ "pushq %%rdi\n\t" \ + /* For TLS access, we have to do function calls. However, \ + * we must not lose the original value of: \ + * rsp, rflags, rax, rbx, rcx, rdx, rsi, rdi, rbp, r8, r9 \ + * r10, r11, r12, r13, r14, r15, cs, ss \ + * \ + * Some registers' original values have already been saved: \ + * rsp, rflags, rax, rbx, rdi \ + * \ + * In addition, the x86-64 ABI says the following registers \ + * belong to the caller function: \ + * rbp, rbx, r12, r13, r14, r15 \ + * \ + * The following registers should not be changed by the callee: \ + * cs, ss \ + * \ + * Therefore, the following registers must be explicitly \ + * preserved: \ + * rcx, rdx, rsi, r8, r9, r10, r11 \ + */ \ + "pushq %%rcx\n\t" \ + "pushq %%rdx\n\t" \ + "pushq %%rsi\n\t" \ + "pushq %%r8\n\t" \ + "pushq %%r9\n\t" \ + "pushq %%r10\n\t" \ + "pushq %%r11\n\t" \ /* Start TLS access of private reg stack pointer */ \ ".byte 0x66\n\t" \ "leaq ust_reg_stack_ptr@tlsgd(%%rip), %%rdi\n\t" \ @@ -212,6 +273,14 @@ struct registers { /* now the pointer to the private stack is in rax. must add stack size so the ptr points to the stack bottom. */ \ "1:\n\t" \ + /* Pop regs that were pushed for function calls */ \ + "popq %%r11\n\t" \ + "popq %%r10\n\t" \ + "popq %%r9\n\t" \ + "popq %%r8\n\t" \ + "popq %%rsi\n\t" \ + "popq %%rdx\n\t" \ + "popq %%rcx\n\t" \ /* Manually push rsp to private stack */ \ "addq $-8,(%%rax)\n\t" \ "movq 32(%%rsp), %%rdi\n\t" \