um: Make stack trace reliable against kernel mode faults

As UML uses an alternative signal stack we cannot use
the current stack pointer for stack dumping if UML itself
dies by SIGSEGV. To bypass this issue we save regs taken
from mcontext in our segv handler into thread_struct and
use these regs to obtain the stack pointer in show_stack().

Signed-off-by: Richard Weinberger <richard@nod.at>

2 files changed, 34 insertions, 12 deletions

diff --git a/arch/um/kernel/sysrq.c b/arch/um/kernel/sysrq.c
index 33cc72e26c6e..7122bf9c753e 100644
--- a/arch/um/kernel/sysrq.c
+++ b/arch/um/kernel/sysrq.c
@@ -12,6 +12,7 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <asm/sysrq.h>
+#include <os.h>
 
 struct stack_frame {
         struct stack_frame *next_frame;
@@ -48,29 +49,42 @@ static void print_stack_trace(unsigned long *sp, unsigned long bp)
 /*Stolen from arch/i386/kernel/traps.c */
 static const int kstack_depth_to_print = 24;
 
-static unsigned long get_frame_pointer(struct task_struct *task)
+static unsigned long get_frame_pointer(struct task_struct *task,
+                                       struct pt_regs *segv_regs)
 {
         if (!task || task == current)
-                return current_bp();
+                return segv_regs ? PT_REGS_BP(segv_regs) : current_bp();
         else
                 return KSTK_EBP(task);
 }
 
+static unsigned long *get_stack_pointer(struct task_struct *task,
+                                        struct pt_regs *segv_regs)
+{
+        if (!task || task == current)
+                return segv_regs ? (unsigned long *)PT_REGS_SP(segv_regs) : current_sp();
+        else
+                return (unsigned long *)KSTK_ESP(task);
+}
+
 void show_stack(struct task_struct *task, unsigned long *stack)
 {
         unsigned long *sp = stack, bp = 0;
+        struct pt_regs *segv_regs = current->thread.segv_regs;
         int i;
 
+        if (!segv_regs && os_is_signal_stack()) {
+                printk(KERN_ERR "Received SIGSEGV in SIGSEGV handler,"
+                                " aborting stack trace!\n");
+                return;
+        }
+
 #ifdef CONFIG_FRAME_POINTER
-        bp = get_frame_pointer(task);
+        bp = get_frame_pointer(task, segv_regs);
 #endif
 
-        if (!stack) {
-                if (!task || task == current)
-                        sp = current_sp();
-                else
-                        sp = (unsigned long *)KSTK_ESP(task);
-        }
+        if (!stack)
+                sp = get_stack_pointer(task, segv_regs);
 
         printk(KERN_INFO "Stack:\n");
         stack = sp;
diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
index 5c3aef74237f..974b87474a99 100644
--- a/arch/um/kernel/trap.c
+++ b/arch/um/kernel/trap.c
@@ -206,9 +206,12 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
         int is_write = FAULT_WRITE(fi);
         unsigned long address = FAULT_ADDRESS(fi);
 
+        if (regs)
+                current->thread.segv_regs = container_of(regs, struct pt_regs, regs);
+
         if (!is_user && (address >= start_vm) && (address < end_vm)) {
                 flush_tlb_kernel_vm();
-                return 0;
+                goto out;
         }
         else if (current->mm == NULL) {
                 show_regs(container_of(regs, struct pt_regs, regs));
@@ -230,7 +233,7 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
 
         catcher = current->thread.fault_catcher;
         if (!err)
-                return 0;
+                goto out;
         else if (catcher != NULL) {
                 current->thread.fault_addr = (void *) address;
                 UML_LONGJMP(catcher, 1);
@@ -238,7 +241,7 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
         else if (current->thread.fault_addr != NULL)
                 panic("fault_addr set but no fault catcher");
         else if (!is_user && arch_fixup(ip, regs))
-                return 0;
+                goto out;
 
         if (!is_user) {
                 show_regs(container_of(regs, struct pt_regs, regs));
@@ -262,6 +265,11 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
                 current->thread.arch.faultinfo = fi;
                 force_sig_info(SIGSEGV, &si, current);
         }
+
+out:
+        if (regs)
+                current->thread.segv_regs = NULL;
+
         return 0;
 }