[PATCH] KVM: selftests: Gracefully handle empty stack traces

[PATCH] KVM: selftests: Gracefully handle empty stack traces

[David Matlack]

Bail out of test_dump_stack() if the stack trace is empty rather than
invoking addr2line with zero addresses. The problem with the latter is
that addr2line will block waiting for addresses to be passed in via
stdin, e.g. if running a selftest from an interactive terminal.

Opportunistically fix up the comment that mentions skipping 3 frames
since only 2 are skipped in the code.

Cc: Vipin Sharma <vipinsh@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 tools/testing/selftests/kvm/lib/assert.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
index 71ade6100fd3..c1ce54a41eca 100644
--- a/tools/testing/selftests/kvm/lib/assert.c
+++ b/tools/testing/selftests/kvm/lib/assert.c
@@ -42,12 +42,18 @@ static void test_dump_stack(void)
 	c = &cmd[0];
 	c += sprintf(c, "%s", addr2line);
 	/*
-	 * Skip the first 3 frames: backtrace, test_dump_stack, and
-	 * test_assert. We hope that backtrace isn't inlined and the other two
-	 * we've declared noinline.
+	 * Skip the first 2 frames, which should be test_dump_stack() and
+	 * test_assert(); both of which are declared noinline.  Bail if the
+	 * resulting stack trace would be empty. Otherwise, addr2line will block
+	 * waiting for addresses to be passed in via stdin.
 	 */
+	if (n <= 2) {
+		fputs("  (stack trace empty)\n", stderr);
+		return;
+	}
 	for (i = 2; i < n; i++)
 		c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
+
 	c += sprintf(c, "%s", pipeline);
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-result"

base-commit: 372d07084593dc7a399bf9bee815711b1fb1bcf2
prerequisite-patch-id: 2e3661ba8856c29b769499bac525b6943d9284b8
prerequisite-patch-id: 1a148d98d96d73a520ed070260608ddf1bdd0f08
-- 
2.37.3.998.g577e59143f-goog

Re: [PATCH] KVM: selftests: Gracefully handle empty stack traces

[Vipin Sharma]

On Thu, Sep 22, 2022 at 4:17 PM David Matlack <dmatlack@google.com> wrote:
>
> Bail out of test_dump_stack() if the stack trace is empty rather than
> invoking addr2line with zero addresses. The problem with the latter is
> that addr2line will block waiting for addresses to be passed in via
> stdin, e.g. if running a selftest from an interactive terminal.
>
> Opportunistically fix up the comment that mentions skipping 3 frames
> since only 2 are skipped in the code.
>
> Cc: Vipin Sharma <vipinsh@google.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Signed-off-by: David Matlack <dmatlack@google.com>
> ---
>  tools/testing/selftests/kvm/lib/assert.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
> index 71ade6100fd3..c1ce54a41eca 100644
> --- a/tools/testing/selftests/kvm/lib/assert.c
> +++ b/tools/testing/selftests/kvm/lib/assert.c
> @@ -42,12 +42,18 @@ static void test_dump_stack(void)
>         c = &cmd[0];
>         c += sprintf(c, "%s", addr2line);
>         /*
> -        * Skip the first 3 frames: backtrace, test_dump_stack, and
> -        * test_assert. We hope that backtrace isn't inlined and the other two
> -        * we've declared noinline.
> +        * Skip the first 2 frames, which should be test_dump_stack() and
> +        * test_assert(); both of which are declared noinline.  Bail if the
> +        * resulting stack trace would be empty. Otherwise, addr2line will block
> +        * waiting for addresses to be passed in via stdin.
>          */
> +       if (n <= 2) {
> +               fputs("  (stack trace empty)\n", stderr);
> +               return;
> +       }

Shouldn't this condition be put immediately after
        n = backtrace(stack,n)

It is more natural to check the return value when an API has returned.

Verified that this change does fix the issue. Thanks for the fix.

>         for (i = 2; i < n; i++)
>                 c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
> +
>         c += sprintf(c, "%s", pipeline);
>  #pragma GCC diagnostic push
>  #pragma GCC diagnostic ignored "-Wunused-result"
>
> base-commit: 372d07084593dc7a399bf9bee815711b1fb1bcf2
> prerequisite-patch-id: 2e3661ba8856c29b769499bac525b6943d9284b8
> prerequisite-patch-id: 1a148d98d96d73a520ed070260608ddf1bdd0f08
> --
> 2.37.3.998.g577e59143f-goog
>

Re: [PATCH] KVM: selftests: Gracefully handle empty stack traces

[Sean Christopherson]

On Mon, Sep 26, 2022, Vipin Sharma wrote:
> On Thu, Sep 22, 2022 at 4:17 PM David Matlack <dmatlack@google.com> wrote:
> >
> > Bail out of test_dump_stack() if the stack trace is empty rather than
> > invoking addr2line with zero addresses. The problem with the latter is
> > that addr2line will block waiting for addresses to be passed in via
> > stdin, e.g. if running a selftest from an interactive terminal.

How does this bug occur?  Does backtrace() get inlined?

> > Opportunistically fix up the comment that mentions skipping 3 frames
> > since only 2 are skipped in the code.
> >
> > Cc: Vipin Sharma <vipinsh@google.com>
> > Cc: Sean Christopherson <seanjc@google.com>
> > Signed-off-by: David Matlack <dmatlack@google.com>
> > ---
> >  tools/testing/selftests/kvm/lib/assert.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
> > index 71ade6100fd3..c1ce54a41eca 100644
> > --- a/tools/testing/selftests/kvm/lib/assert.c
> > +++ b/tools/testing/selftests/kvm/lib/assert.c
> > @@ -42,12 +42,18 @@ static void test_dump_stack(void)
> >         c = &cmd[0];
> >         c += sprintf(c, "%s", addr2line);
> >         /*
> > -        * Skip the first 3 frames: backtrace, test_dump_stack, and
> > -        * test_assert. We hope that backtrace isn't inlined and the other two
> > -        * we've declared noinline.
> > +        * Skip the first 2 frames, which should be test_dump_stack() and
> > +        * test_assert(); both of which are declared noinline.  Bail if the
> > +        * resulting stack trace would be empty. Otherwise, addr2line will block
> > +        * waiting for addresses to be passed in via stdin.
> >          */
> > +       if (n <= 2) {
> > +               fputs("  (stack trace empty)\n", stderr);
> > +               return;
> > +       }
> 
> Shouldn't this condition be put immediately after
>         n = backtrace(stack,n)

Agreed, that would be more intuitive.

Re: [PATCH] KVM: selftests: Gracefully handle empty stack traces

[David Matlack]

On Mon, Sep 26, 2022 at 08:53:35PM +0000, Sean Christopherson wrote:
> On Mon, Sep 26, 2022, Vipin Sharma wrote:
> > On Thu, Sep 22, 2022 at 4:17 PM David Matlack <dmatlack@google.com> wrote:
> > >
> > > Bail out of test_dump_stack() if the stack trace is empty rather than
> > > invoking addr2line with zero addresses. The problem with the latter is
> > > that addr2line will block waiting for addresses to be passed in via
> > > stdin, e.g. if running a selftest from an interactive terminal.
> 
> How does this bug occur?  Does backtrace() get inlined?

backtrace() is returning 0. I haven't debugged it further than that yet.
I figured gracefully handling an empty stack trace would be useful to
have independent of this specific issue (which I assume has something to
do with our Google-specific build process).

backtrace() is not getting inlined.

> 
> > > Opportunistically fix up the comment that mentions skipping 3 frames
> > > since only 2 are skipped in the code.
> > >
> > > Cc: Vipin Sharma <vipinsh@google.com>
> > > Cc: Sean Christopherson <seanjc@google.com>
> > > Signed-off-by: David Matlack <dmatlack@google.com>
> > > ---
> > >  tools/testing/selftests/kvm/lib/assert.c | 12 +++++++++---
> > >  1 file changed, 9 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
> > > index 71ade6100fd3..c1ce54a41eca 100644
> > > --- a/tools/testing/selftests/kvm/lib/assert.c
> > > +++ b/tools/testing/selftests/kvm/lib/assert.c
> > > @@ -42,12 +42,18 @@ static void test_dump_stack(void)
> > >         c = &cmd[0];
> > >         c += sprintf(c, "%s", addr2line);
> > >         /*
> > > -        * Skip the first 3 frames: backtrace, test_dump_stack, and
> > > -        * test_assert. We hope that backtrace isn't inlined and the other two
> > > -        * we've declared noinline.
> > > +        * Skip the first 2 frames, which should be test_dump_stack() and
> > > +        * test_assert(); both of which are declared noinline.  Bail if the
> > > +        * resulting stack trace would be empty. Otherwise, addr2line will block
> > > +        * waiting for addresses to be passed in via stdin.
> > >          */
> > > +       if (n <= 2) {
> > > +               fputs("  (stack trace empty)\n", stderr);
> > > +               return;
> > > +       }
> > 
> > Shouldn't this condition be put immediately after
> >         n = backtrace(stack,n)
> 
> Agreed, that would be more intuitive.

I had that at one point, but then it became confusing that the check is for
(n <= 2) and not (!n).

How about this?

diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
index 71ade6100fd3..2b56bbff970c 100644
--- a/tools/testing/selftests/kvm/lib/assert.c
+++ b/tools/testing/selftests/kvm/lib/assert.c
@@ -38,16 +38,28 @@ static void test_dump_stack(void)
                 1];
        char *c;

-       n = backtrace(stack, n);
        c = &cmd[0];
        c += sprintf(c, "%s", addr2line);
-       /*
-        * Skip the first 3 frames: backtrace, test_dump_stack, and
-        * test_assert. We hope that backtrace isn't inlined and the other two
-        * we've declared noinline.
-        */
-       for (i = 2; i < n; i++)
-               c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
+
+       n = backtrace(stack, n);
+       if (n > 2) {
+               /*
+                * Skip the first 2 frames, which should be test_dump_stack()
+                * and test_assert(); both of which are declared noinline.
+                */
+               for (i = 2; i < n; i++)
+                       c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
+       } else {
+               /*
+                * Bail if the resulting stack trace would be empty. Otherwise,
+                * addr2line will block waiting for addresses to be passed in
+                * via stdin.
+                */
+               fputs("  (stack trace missing)\n", stderr);
+               return;
+       }
+
        c += sprintf(c, "%s", pipeline);
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wunused-result"

Re: [PATCH] KVM: selftests: Gracefully handle empty stack traces

[Paolo Bonzini]

On 9/26/22 23:47, David Matlack wrote:
> How about this?
> 
> diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
> index 71ade6100fd3..2b56bbff970c 100644
> --- a/tools/testing/selftests/kvm/lib/assert.c
> +++ b/tools/testing/selftests/kvm/lib/assert.c
> @@ -38,16 +38,28 @@ static void test_dump_stack(void)
>                   1];
>          char *c;
> 
> -       n = backtrace(stack, n);
>          c = &cmd[0];
>          c += sprintf(c, "%s", addr2line);
> -       /*
> -        * Skip the first 3 frames: backtrace, test_dump_stack, and
> -        * test_assert. We hope that backtrace isn't inlined and the other two
> -        * we've declared noinline.
> -        */
> -       for (i = 2; i < n; i++)
> -               c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
> +
> +       n = backtrace(stack, n);
> +       if (n > 2) {
> +               /*
> +                * Skip the first 2 frames, which should be test_dump_stack()
> +                * and test_assert(); both of which are declared noinline.
> +                */
> +               for (i = 2; i < n; i++)
> +                       c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
> +       } else {
> +               /*
> +                * Bail if the resulting stack trace would be empty. Otherwise,
> +                * addr2line will block waiting for addresses to be passed in
> +                * via stdin.
> +                */
> +               fputs("  (stack trace missing)\n", stderr);
> +               return;
> +       }
> +
>          c += sprintf(c, "%s", pipeline);
>   #pragma GCC diagnostic push
>   #pragma GCC diagnostic ignored "-Wunused-result"
> 

I think your original patch is better, just with

diff --git a/tools/testing/selftests/kvm/lib/assert.c b/tools/testing/selftests/kvm/lib/assert.c
index c1ce54a41eca..be2cab00f541 100644
--- a/tools/testing/selftests/kvm/lib/assert.c
+++ b/tools/testing/selftests/kvm/lib/assert.c
@@ -36,11 +36,9 @@ static void test_dump_stack(void)
  		 n * (((sizeof(void *)) * 2) + 1) +
  		 /* Null terminator: */
  		 1];
-	char *c;
+	char *c = cmd;
  
  	n = backtrace(stack, n);
-	c = &cmd[0];
-	c += sprintf(c, "%s", addr2line);
  	/*
  	 * Skip the first 2 frames, which should be test_dump_stack() and
  	 * test_assert(); both of which are declared noinline.  Bail if the
@@ -51,6 +49,8 @@ static void test_dump_stack(void)
  		fputs("  (stack trace empty)\n", stderr);
  		return;
  	}
+
+	c += sprintf(c, "%s", addr2line);
  	for (i = 2; i < n; i++)
  		c += sprintf(c, " %lx", ((unsigned long) stack[i]) - 1);
  

squashed in to keep the "if" and backtrace() call as close as possible.

Paolo