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SubjectRe: [PATCH] prctl_set_mm: downgrade mmap_sem to read lock
On Thu 18-04-19 15:50:39, Michal Koutny wrote:
> I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls
> [1], so at least downgrade the write acquisition of mmap_sem as in the
> patch below (that should be stacked on the previous one or squashed).
>
> Cyrill, you mentioned lock changes in [1] but the link seems empty. Is
> it supposed to be [2]? That could be an alternative to this patch after
> some refreshments and clarifications.
>
>
> [1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/
> [2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/
>
> ========
>
> Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
> arg_start|end and env_start|end in mm_struct") we use arg_lock for
> boundaries modifications. Synchronize prctl_set_mm with this lock and
> keep mmap_sem for reading only (analogous to what we already do in
> prctl_set_mm_map).
>
> Also, save few cycles by looking up VMA only after performing basic
> arguments validation.
>
> Signed-off-by: Michal Koutný <mkoutny@suse.com>

Looks good to me. Please send both patches in one series once you get a
review feedback from other people.

> ---
> kernel/sys.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 12df0e5434b8..bbce0f26d707 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr,
>
> error = -EINVAL;
>
> - down_write(&mm->mmap_sem);
> - vma = find_vma(mm, addr);
> + /*
> + * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
> + * a) concurrent sys_brk, b) finding VMA for addr validation.
> + */
> + down_read(&mm->mmap_sem);
> + spin_lock(&mm->arg_lock);
>
> prctl_map.start_code = mm->start_code;
> prctl_map.end_code = mm->end_code;
> @@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
> if (error)
> goto out;
>
> + vma = find_vma(mm, addr);
> switch (opt) {
> /*
> * If command line arguments and environment
> @@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
>
> error = 0;
> out:
> - up_write(&mm->mmap_sem);
> + spin_unlock(&mm->arg_lock);
> + up_read(&mm->mmap_sem);
> return error;
> }
>
> --
> 2.16.4

--
Michal Hocko
SUSE Labs

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