Re: [PATCH 06/10] mm: update coredump logic to correctly use bitmap mm flags

[Mike Rapoport]

On Tue, Aug 12, 2025 at 04:44:15PM +0100, Lorenzo Stoakes wrote:
> The coredump logic is slightly different from other users in that it both
> stores mm flags and additionally sets and gets using masks.
> 
> Since the MMF_DUMPABLE_* flags must remain as they are for uABI reasons,
> and of course these are within the first 32-bits of the flags, it is
> reasonable to provide access to these in the same fashion so this logic can
> all still keep working as it has been.
> 
> Therefore, introduce coredump-specific helpers __mm_flags_get_dumpable()
> and __mm_flags_set_mask_dumpable() for this purpose, and update all core
> dump users of mm flags to use these.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>

> ---
>  fs/coredump.c                  |  4 +++-
>  fs/exec.c                      |  2 +-
>  fs/pidfs.c                     |  7 +++++--
>  fs/proc/base.c                 |  8 +++++---
>  include/linux/sched/coredump.h | 21 ++++++++++++++++++++-
>  5 files changed, 34 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/coredump.c b/fs/coredump.c
> index fedbead956ed..e5d9d6276990 100644
> --- a/fs/coredump.c
> +++ b/fs/coredump.c
> @@ -1103,8 +1103,10 @@ void vfs_coredump(const kernel_siginfo_t *siginfo)
>  		 * We must use the same mm->flags while dumping core to avoid
>  		 * inconsistency of bit flags, since this flag is not protected
>  		 * by any locks.
> +		 *
> +		 * Note that we only care about MMF_DUMP* flags.
>  		 */
> -		.mm_flags = mm->flags,
> +		.mm_flags = __mm_flags_get_dumpable(mm),
>  		.vma_meta = NULL,
>  		.cpu = raw_smp_processor_id(),
>  	};
> diff --git a/fs/exec.c b/fs/exec.c
> index 2a1e5e4042a1..dbac0e84cc3e 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1999,7 +1999,7 @@ void set_dumpable(struct mm_struct *mm, int value)
>  	if (WARN_ON((unsigned)value > SUID_DUMP_ROOT))
>  		return;
>  
> -	set_mask_bits(&mm->flags, MMF_DUMPABLE_MASK, value);
> +	__mm_flags_set_mask_dumpable(mm, value);
>  }
>  
>  SYSCALL_DEFINE3(execve,
> diff --git a/fs/pidfs.c b/fs/pidfs.c
> index edc35522d75c..5148b7646b7f 100644
> --- a/fs/pidfs.c
> +++ b/fs/pidfs.c
> @@ -357,8 +357,11 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
>  
>  	if ((kinfo.mask & PIDFD_INFO_COREDUMP) && !(kinfo.coredump_mask)) {
>  		task_lock(task);
> -		if (task->mm)
> -			kinfo.coredump_mask = pidfs_coredump_mask(task->mm->flags);
> +		if (task->mm) {
> +			unsigned long flags = __mm_flags_get_dumpable(task->mm);
> +
> +			kinfo.coredump_mask = pidfs_coredump_mask(flags);
> +		}
>  		task_unlock(task);
>  	}
>  
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 62d35631ba8c..f0c093c58aaf 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -2962,8 +2962,10 @@ static ssize_t proc_coredump_filter_read(struct file *file, char __user *buf,
>  	ret = 0;
>  	mm = get_task_mm(task);
>  	if (mm) {
> +		unsigned long flags = __mm_flags_get_dumpable(mm);
> +
>  		len = snprintf(buffer, sizeof(buffer), "%08lx\n",
> -			       ((mm->flags & MMF_DUMP_FILTER_MASK) >>
> +			       ((flags & MMF_DUMP_FILTER_MASK) >>
>  				MMF_DUMP_FILTER_SHIFT));
>  		mmput(mm);
>  		ret = simple_read_from_buffer(buf, count, ppos, buffer, len);
> @@ -3002,9 +3004,9 @@ static ssize_t proc_coredump_filter_write(struct file *file,
>  
>  	for (i = 0, mask = 1; i < MMF_DUMP_FILTER_BITS; i++, mask <<= 1) {
>  		if (val & mask)
> -			set_bit(i + MMF_DUMP_FILTER_SHIFT, &mm->flags);
> +			mm_flags_set(i + MMF_DUMP_FILTER_SHIFT, mm);
>  		else
> -			clear_bit(i + MMF_DUMP_FILTER_SHIFT, &mm->flags);
> +			mm_flags_clear(i + MMF_DUMP_FILTER_SHIFT, mm);
>  	}
>  
>  	mmput(mm);
> diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
> index 6eb65ceed213..19ecfcceb27a 100644
> --- a/include/linux/sched/coredump.h
> +++ b/include/linux/sched/coredump.h
> @@ -2,12 +2,29 @@
>  #ifndef _LINUX_SCHED_COREDUMP_H
>  #define _LINUX_SCHED_COREDUMP_H
>  
> +#include <linux/compiler_types.h>
>  #include <linux/mm_types.h>
>  
>  #define SUID_DUMP_DISABLE	0	/* No setuid dumping */
>  #define SUID_DUMP_USER		1	/* Dump as user of process */
>  #define SUID_DUMP_ROOT		2	/* Dump as root */
>  
> +static inline unsigned long __mm_flags_get_dumpable(struct mm_struct *mm)
> +{
> +	/*
> +	 * By convention, dumpable bits are contained in first 32 bits of the
> +	 * bitmap, so we can simply access this first unsigned long directly.
> +	 */
> +	return __mm_flags_get_word(mm);
> +}
> +
> +static inline void __mm_flags_set_mask_dumpable(struct mm_struct *mm, int value)
> +{
> +	unsigned long *bitmap = ACCESS_PRIVATE(&mm->_flags, __mm_flags);
> +
> +	set_mask_bits(bitmap, MMF_DUMPABLE_MASK, value);
> +}
> +
>  extern void set_dumpable(struct mm_struct *mm, int value);
>  /*
>   * This returns the actual value of the suid_dumpable flag. For things
> @@ -22,7 +39,9 @@ static inline int __get_dumpable(unsigned long mm_flags)
>  
>  static inline int get_dumpable(struct mm_struct *mm)
>  {
> -	return __get_dumpable(mm->flags);
> +	unsigned long flags = __mm_flags_get_dumpable(mm);
> +
> +	return __get_dumpable(flags);
>  }
>  
>  #endif /* _LINUX_SCHED_COREDUMP_H */
> -- 
> 2.50.1
> 

-- 
Sincerely yours,
Mike.