x86,kgdb: Fix DEBUG_RODATA limitation using text_poke()

There has long been a limitation using software breakpoints with a
kernel compiled with CONFIG_DEBUG_RODATA going back to 2.6.26. For
this particular patch, it will apply cleanly and has been tested all
the way back to 2.6.36.

The kprobes code uses the text_poke() function which accommodates
writing a breakpoint into a read-only page.  The x86 kgdb code can
solve the problem similarly by overriding the default breakpoint
set/remove routines and using text_poke() directly.

The x86 kgdb code will first attempt to use the traditional
probe_kernel_write(), and next try using a the text_poke() function.
The break point install method is tracked such that the correct break
point removal routine will get called later on.

Cc: x86@kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: stable@vger.kernel.org # >= 2.6.36
Inspried-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index fdc37b3..b9bd9d8 100644 (file)
--- a/arch/x86/kernel/kgdb.c
+++ b/arch/x86/kernel/kgdb.c
@@ -43,6 +43,8 @@
 #include <linux/smp.h>
 #include <linux/nmi.h>
 #include <linux/hw_breakpoint.h>
+#include <linux/uaccess.h>
+#include <linux/memory.h>
 
 #include <asm/debugreg.h>
 #include <asm/apicdef.h>
@@ -742,6 +744,64 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
        regs->ip = ip;
 }
 
+int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
+{
+       int err;
+       char opc[BREAK_INSTR_SIZE];
+
+       bpt->type = BP_BREAKPOINT;
+       err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
+                               BREAK_INSTR_SIZE);
+       if (err)
+               return err;
+       err = probe_kernel_write((char *)bpt->bpt_addr,
+                                arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
+#ifdef CONFIG_DEBUG_RODATA
+       if (!err)
+               return err;
+       /*
+        * It is safe to call text_poke() because normal kernel execution
+        * is stopped on all cores, so long as the text_mutex is not locked.
+        */
+       if (mutex_is_locked(&text_mutex))
+               return -EBUSY;
+       text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
+                 BREAK_INSTR_SIZE);
+       err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
+       if (err)
+               return err;
+       if (memcmp(opc, arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE))
+               return -EINVAL;
+       bpt->type = BP_POKE_BREAKPOINT;
+#endif /* CONFIG_DEBUG_RODATA */
+       return err;
+}
+
+int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
+{
+#ifdef CONFIG_DEBUG_RODATA
+       int err;
+       char opc[BREAK_INSTR_SIZE];
+
+       if (bpt->type != BP_POKE_BREAKPOINT)
+               goto knl_write;
+       /*
+        * It is safe to call text_poke() because normal kernel execution
+        * is stopped on all cores, so long as the text_mutex is not locked.
+        */
+       if (mutex_is_locked(&text_mutex))
+               goto knl_write;
+       text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
+       err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
+       if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
+               goto knl_write;
+       return err;
+knl_write:
+#endif /* CONFIG_DEBUG_RODATA */
+       return probe_kernel_write((char *)bpt->bpt_addr,
+                                 (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
+}
+
 struct kgdb_arch arch_kgdb_ops = {
        /* Breakpoint instruction: */
        .gdb_bpt_instr          = { 0xcc },

diff --git a/drivers/misc/kgdbts.c b/drivers/misc/kgdbts.c
index d087456..3aa9a96 100644 (file)
--- a/drivers/misc/kgdbts.c
+++ b/drivers/misc/kgdbts.c
@@ -968,22 +968,6 @@ static void run_singlestep_break_test(void)
        kgdbts_break_test();
 }
 
-static void test_debug_rodata(void)
-{
-#ifdef CONFIG_DEBUG_RODATA
-       /* Until there is an api to write to read-only text segments, use
-        * HW breakpoints for the remainder of any tests, else print a
-        * failure message if hw breakpoints do not work.
-        */
-       if (!(arch_kgdb_ops.flags & KGDB_HW_BREAKPOINT && hwbreaks_ok)) {
-               eprintk("kgdbts: HW breakpoints BROKEN, ending tests\n");
-               return;
-       }
-       force_hwbrks = 1;
-       v1printk("kgdbts:Using HW breakpoints for SW breakpoint tests\n");
-#endif /* CONFIG_DEBUG_RODATA */
-}
-
 static void kgdbts_run_tests(void)
 {
        char *ptr;
@@ -1016,7 +1000,6 @@ static void kgdbts_run_tests(void)
                v1printk("kgdbts:RUN access write breakpoint test\n");
                run_hw_break_test(0);
        }
-       test_debug_rodata();
 
        /* required internal KGDB tests */
        v1printk("kgdbts:RUN plant and detach test\n");

diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index e5d689c..c4d2fc1 100644 (file)
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -63,7 +63,8 @@ enum kgdb_bptype {
        BP_HARDWARE_BREAKPOINT,
        BP_WRITE_WATCHPOINT,
        BP_READ_WATCHPOINT,
-       BP_ACCESS_WATCHPOINT
+       BP_ACCESS_WATCHPOINT,
+       BP_POKE_BREAKPOINT,
 };
 
 enum kgdb_bpstate {