Replace simple_strtoul() with the recommended kstrtoul() for parsing the
'coredump_filter=' boot parameter.

Check the return value of kstrtoul() and reject invalid values. This
adds error handling while preserving behavior for existing values, and
removes use of the deprecated simple_strtoul() helper. The current code
silently sets 'default_dump_filter = 0' if parsing fails, instead of
leaving the default value (MMF_DUMP_FILTER_DEFAULT) unchanged.

Rename the static variable 'default_dump_filter' to 'coredump_filter'
since it does not necessarily contain the default value and the current
name can be misleading.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
---
 kernel/fork.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index b1f3915d5f8e..f33ee7fe53ad 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1012,13 +1012,14 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 
 __cacheline_aligned_in_smp DEFINE_SPINLOCK(mmlist_lock);
 
-static unsigned long default_dump_filter = MMF_DUMP_FILTER_DEFAULT;
+static unsigned long coredump_filter = MMF_DUMP_FILTER_DEFAULT;
 
 static int __init coredump_filter_setup(char *s)
 {
-	default_dump_filter =
-		(simple_strtoul(s, NULL, 0) << MMF_DUMP_FILTER_SHIFT) &
-		MMF_DUMP_FILTER_MASK;
+	if (kstrtoul(s, 0, &coredump_filter))
+		return 0;
+	coredump_filter <<= MMF_DUMP_FILTER_SHIFT;
+	coredump_filter &= MMF_DUMP_FILTER_MASK;
 	return 1;
 }
 
@@ -1104,7 +1105,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
 		__mm_flags_overwrite_word(mm, mmf_init_legacy_flags(flags));
 		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
 	} else {
-		__mm_flags_overwrite_word(mm, default_dump_filter);
+		__mm_flags_overwrite_word(mm, coredump_filter);
 		mm->def_flags = 0;
 	}
 
-- 
Thorsten Blum <thorsten.blum@linux.dev>
GPG: 1D60 735E 8AEF 3BE4 73B6  9D84 7336 78FD 8DFE EAD4

On Mon, Dec 15, 2025 at 03:21:52PM +0100, Thorsten Blum wrote:
> Replace simple_strtoul() with the recommended kstrtoul() for parsing the
> 'coredump_filter=' boot parameter.
> 
> Check the return value of kstrtoul() and reject invalid values. This
> adds error handling while preserving behavior for existing values, and
> removes use of the deprecated simple_strtoul() helper. The current code
> silently sets 'default_dump_filter = 0' if parsing fails, instead of
> leaving the default value (MMF_DUMP_FILTER_DEFAULT) unchanged.
> 
> Rename the static variable 'default_dump_filter' to 'coredump_filter'
> since it does not necessarily contain the default value and the current
> name can be misleading.
> 
> Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

> ---
>  kernel/fork.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b1f3915d5f8e..f33ee7fe53ad 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1012,13 +1012,14 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>  
>  __cacheline_aligned_in_smp DEFINE_SPINLOCK(mmlist_lock);
>  
> -static unsigned long default_dump_filter = MMF_DUMP_FILTER_DEFAULT;
> +static unsigned long coredump_filter = MMF_DUMP_FILTER_DEFAULT;
>  
>  static int __init coredump_filter_setup(char *s)
>  {
> -	default_dump_filter =
> -		(simple_strtoul(s, NULL, 0) << MMF_DUMP_FILTER_SHIFT) &
> -		MMF_DUMP_FILTER_MASK;
> +	if (kstrtoul(s, 0, &coredump_filter))
> +		return 0;
> +	coredump_filter <<= MMF_DUMP_FILTER_SHIFT;
> +	coredump_filter &= MMF_DUMP_FILTER_MASK;
>  	return 1;
>  }
>  
> @@ -1104,7 +1105,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
>  		__mm_flags_overwrite_word(mm, mmf_init_legacy_flags(flags));
>  		mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
>  	} else {
> -		__mm_flags_overwrite_word(mm, default_dump_filter);
> +		__mm_flags_overwrite_word(mm, coredump_filter);
>  		mm->def_flags = 0;
>  	}
>  
> -- 
> Thorsten Blum <thorsten.blum@linux.dev>
> GPG: 1D60 735E 8AEF 3BE4 73B6  9D84 7336 78FD 8DFE EAD4
> 

-- 
Sincerely yours,
Mike.