Currently kgdbts can get stuck waiting for do_sys_open() to be called
in some of the current tests. This is because C compilers often
automatically inline this function, which is a very thin wrapper around
do_sys_openat2(), into some of its callers. gcc-10 does this on (at least)
both x86 and arm64.
We can fix the test suite by placing the breakpoints on do_sys_openat2()
instead since that isn't (currently) inlined. However do_sys_openat2() is
a static function so we cannot simply use an addressof. Since we are
testing debug machinery it is acceptable to use kallsyms to lookup a
suitable address because this is more or less what kdb does in the same
circumstances. Re-implement lookup_addr() to be based on kallsyms rather
than function pointers.
Signed-off-by: Daniel Thompson <[email protected]>
---
Notes:
So less than a week ago I said I had nothing pending for kgdbts.
That was entirely true when I said it... but then this came up.
drivers/misc/kgdbts.c | 48 ++++++++++++++++++++++++++-----------------
1 file changed, 29 insertions(+), 19 deletions(-)
diff --git a/drivers/misc/kgdbts.c b/drivers/misc/kgdbts.c
index 945701bce5536..2679374ca8361 100644
--- a/drivers/misc/kgdbts.c
+++ b/drivers/misc/kgdbts.c
@@ -92,6 +92,7 @@
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/sched/task.h>
+#include <linux/kallsyms.h>
#include <asm/sections.h>
@@ -200,21 +201,30 @@ static noinline void kgdbts_break_test(void)
v2printk("kgdbts: breakpoint complete\n");
}
-/* Lookup symbol info in the kernel */
+/*
+ * This is a cached wrapper for kallsyms_lookup_name().
+ *
+ * The cache is a big win for several tests. For example it more the doubles
+ * the cycles per second during the sys_open test. This is not theoretic,
+ * the performance improvement shows up at human scale, especially when
+ * testing using emulators.
+ *
+ * Obviously neither re-entrant nor thread-safe but that is OK since it
+ * can only be called from the debug trap (and therefore all other CPUs
+ * are halted).
+ */
static unsigned long lookup_addr(char *arg)
{
- unsigned long addr = 0;
-
- if (!strcmp(arg, "kgdbts_break_test"))
- addr = (unsigned long)kgdbts_break_test;
- else if (!strcmp(arg, "sys_open"))
- addr = (unsigned long)do_sys_open;
- else if (!strcmp(arg, "kernel_clone"))
- addr = (unsigned long)kernel_clone;
- else if (!strcmp(arg, "hw_break_val"))
- addr = (unsigned long)&hw_break_val;
- addr = (unsigned long) dereference_function_descriptor((void *)addr);
- return addr;
+ static char cached_arg[KSYM_NAME_LEN];
+ static unsigned long cached_addr;
+
+ if (strcmp(arg, cached_arg)) {
+ strscpy(cached_arg, arg, KSYM_NAME_LEN);
+ cached_addr = kallsyms_lookup_name(arg);
+ }
+
+ return (unsigned long)dereference_function_descriptor(
+ (void *)cached_addr);
}
static void break_helper(char *bp_type, char *arg, unsigned long vaddr)
@@ -310,7 +320,7 @@ static int check_and_rewind_pc(char *put_str, char *arg)
if (arch_needs_sstep_emulation && sstep_addr &&
ip + offset == sstep_addr &&
- ((!strcmp(arg, "sys_open") || !strcmp(arg, "kernel_clone")))) {
+ ((!strcmp(arg, "do_sys_openat2") || !strcmp(arg, "kernel_clone")))) {
/* This is special case for emulated single step */
v2printk("Emul: rewind hit single step bp\n");
restart_from_top_after_write = 1;
@@ -619,14 +629,14 @@ static struct test_struct do_kernel_clone_test[] = {
*/
static struct test_struct sys_open_test[] = {
{ "?", "S0*" }, /* Clear break points */
- { "sys_open", "OK", sw_break, }, /* set sw breakpoint */
+ { "do_sys_openat2", "OK", sw_break, }, /* set sw breakpoint */
{ "c", "T0*", NULL, get_thread_id_continue }, /* Continue */
- { "sys_open", "OK", sw_rem_break }, /*remove breakpoint */
- { "g", "sys_open", NULL, check_and_rewind_pc }, /* check location */
+ { "do_sys_openat2", "OK", sw_rem_break }, /*remove breakpoint */
+ { "g", "do_sys_openat2", NULL, check_and_rewind_pc }, /* check location */
{ "write", "OK", write_regs, emul_reset }, /* Write registers */
{ "s", "T0*", emul_sstep_get, emul_sstep_put }, /* Single step */
- { "g", "sys_open", NULL, check_single_step },
- { "sys_open", "OK", sw_break, }, /* set sw breakpoint */
+ { "g", "do_sys_openat2", NULL, check_single_step },
+ { "do_sys_openat2", "OK", sw_break, }, /* set sw breakpoint */
{ "7", "T0*", skip_back_repeat_test }, /* Loop based on repeat_test */
{ "D", "OK", NULL, final_ack_set }, /* detach and unregister I/O */
{ "", "", get_cont_catch, put_cont_catch },
base-commit: 1e28eed17697bcf343c6743f0028cc3b5dd88bf0
--
2.29.2
On Thu, Mar 25, 2021 at 09:48:07AM +0000, Daniel Thompson wrote:
> Currently kgdbts can get stuck waiting for do_sys_open() to be called
> in some of the current tests. This is because C compilers often
> automatically inline this function, which is a very thin wrapper around
> do_sys_openat2(), into some of its callers. gcc-10 does this on (at least)
> both x86 and arm64.
>
> We can fix the test suite by placing the breakpoints on do_sys_openat2()
> instead since that isn't (currently) inlined. However do_sys_openat2() is
> a static function so we cannot simply use an addressof. Since we are
> testing debug machinery it is acceptable to use kallsyms to lookup a
> suitable address because this is more or less what kdb does in the same
> circumstances. Re-implement lookup_addr() to be based on kallsyms rather
> than function pointers.
>
> Signed-off-by: Daniel Thompson <[email protected]>
> ---
>
> Notes:
> So less than a week ago I said I had nothing pending for kgdbts.
> That was entirely true when I said it... but then this came up.
No worries, I'll take this too :)
thanks,
greg k-h