2021-01-22 09:50:08

by Sumit Garg

Subject: [PATCH] kdb: Make memory allocations more robust

Currently kdb uses in_interrupt() to determine whether it's library
code has been called from the kgdb trap handler or from a saner calling
context such as driver init. This approach is broken because
in_interrupt() alone isn't able to determine kgdb trap handler entry via
normal task context such as [1].

We can improve this by adding check for in_dbg_master() which explicitly
determines if we are running in debugger context. Also, use in_atomic()
instead of in_interrupt() as the former is more appropriate to know atomic
context and moreover the later one is deprecated.

[1] $ echo g > /proc/sysrq-trigger

Signed-off-by: Sumit Garg <[email protected]>
---
kernel/debug/kdb/kdb_private.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
index 7a4a181..7a9ebd9 100644
--- a/kernel/debug/kdb/kdb_private.h
+++ b/kernel/debug/kdb/kdb_private.h
@@ -231,7 +231,7 @@ extern struct task_struct *kdb_curr_task(int);

#define kdb_task_has_cpu(p) (task_curr(p))

-#define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
+#define GFP_KDB (in_atomic() || in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)

extern void *debug_kmalloc(size_t size, gfp_t flags);
extern void debug_kfree(void *);
--
2.7.4

2021-01-22 10:30:58

by Sumit Garg

Subject: Re: [PATCH] kdb: Make memory allocations more robust

On Fri, 22 Jan 2021 at 15:18, Daniel Thompson
<[email protected]> wrote:
>
> On Fri, Jan 22, 2021 at 03:08:31PM +0530, Sumit Garg wrote:
> > Currently kdb uses in_interrupt() to determine whether it's library
> > code has been called from the kgdb trap handler or from a saner calling
> > context such as driver init. This approach is broken because
> > in_interrupt() alone isn't able to determine kgdb trap handler entry via
> > normal task context such as [1].
> >
> > We can improve this by adding check for in_dbg_master() which explicitly
> > determines if we are running in debugger context. Also, use in_atomic()
> > instead of in_interrupt() as the former is more appropriate to know atomic
> > context and moreover the later one is deprecated.
>
> Why do we need the in_atomic() here? Or put another way, why isn't
> in_dbg_master() sufficient?
>

Yes, you are right in_atomic() is redundant after looking at usage of
GFP_KDB. Will get rid of it in v2.

-Sumit

>
> Daniel.
>
>
> >
> > [1] $ echo g > /proc/sysrq-trigger
> >
> > Signed-off-by: Sumit Garg <[email protected]>
> > ---
> > kernel/debug/kdb/kdb_private.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
> > index 7a4a181..7a9ebd9 100644
> > --- a/kernel/debug/kdb/kdb_private.h
> > +++ b/kernel/debug/kdb/kdb_private.h
> > @@ -231,7 +231,7 @@ extern struct task_struct *kdb_curr_task(int);
> >
> > #define kdb_task_has_cpu(p) (task_curr(p))
> >
> > -#define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
> > +#define GFP_KDB (in_atomic() || in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)
> >
> > extern void *debug_kmalloc(size_t size, gfp_t flags);
> > extern void debug_kfree(void *);
> > --
> > 2.7.4
> >

2021-01-22 10:33:52

by Daniel Thompson

Subject: Re: [PATCH] kdb: Make memory allocations more robust

On Fri, Jan 22, 2021 at 03:08:31PM +0530, Sumit Garg wrote:
> Currently kdb uses in_interrupt() to determine whether it's library
> code has been called from the kgdb trap handler or from a saner calling
> context such as driver init. This approach is broken because
> in_interrupt() alone isn't able to determine kgdb trap handler entry via
> normal task context such as [1].
>
> We can improve this by adding check for in_dbg_master() which explicitly
> determines if we are running in debugger context. Also, use in_atomic()
> instead of in_interrupt() as the former is more appropriate to know atomic
> context and moreover the later one is deprecated.

Why do we need the in_atomic() here? Or put another way, why isn't
in_dbg_master() sufficient?


Daniel.


>
> [1] $ echo g > /proc/sysrq-trigger
>
> Signed-off-by: Sumit Garg <[email protected]>
> ---
> kernel/debug/kdb/kdb_private.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/debug/kdb/kdb_private.h b/kernel/debug/kdb/kdb_private.h
> index 7a4a181..7a9ebd9 100644
> --- a/kernel/debug/kdb/kdb_private.h
> +++ b/kernel/debug/kdb/kdb_private.h
> @@ -231,7 +231,7 @@ extern struct task_struct *kdb_curr_task(int);
>
> #define kdb_task_has_cpu(p) (task_curr(p))
>
> -#define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
> +#define GFP_KDB (in_atomic() || in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)
>
> extern void *debug_kmalloc(size_t size, gfp_t flags);
> extern void debug_kfree(void *);
> --
> 2.7.4
>