Re: [PATCH 2/7] pstore: locking: dont lock unless caller asks to

[Kees Cook <keescook@chromium.org>]

On Fri, Oct 7, 2016 at 10:28 PM, Joel Fernandes <joelaf@google.com> wrote:
> In preparation of not locking at all for certain buffers depending on if
> there's contention, make locking optional depending if caller requested it.

This should be a bool argument (or enum), with named values so it's
more readable. For example, these don't immediately tell me what the
argument does:

persistent_ram_write(cxt->cprz, buf, size, 1);
persistent_ram_write(cxt->cprz, buf, size, true);

But this does:

persistent_ram_write(cxt->cprz, buf, size, PSTORE_REQUIRE_LOCKING);

As part of this, can you document in the pstore structure which types
of front-ends require locking and if they don't, why?

-Kees

>
> Signed-off-by: Joel Fernandes <joelaf@google.com>
> ---
>  fs/pstore/ram.c            | 10 +++++-----
>  fs/pstore/ram_core.c       | 27 ++++++++++++++++-----------
>  include/linux/pstore_ram.h |  2 +-
>  3 files changed, 22 insertions(+), 17 deletions(-)
>
> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> index f1219af..751197d 100644
> --- a/fs/pstore/ram.c
> +++ b/fs/pstore/ram.c
> @@ -260,7 +260,7 @@ static size_t ramoops_write_kmsg_hdr(struct persistent_ram_zone *prz,
>                 compressed ? 'C' : 'D');
>         WARN_ON_ONCE(!hdr);
>         len = hdr ? strlen(hdr) : 0;
> -       persistent_ram_write(prz, hdr, len);
> +       persistent_ram_write(prz, hdr, len, 1);
>         kfree(hdr);
>
>         return len;
> @@ -280,17 +280,17 @@ static int notrace ramoops_pstore_write_buf(enum pstore_type_id type,
>         if (type == PSTORE_TYPE_CONSOLE) {
>                 if (!cxt->cprz)
>                         return -ENOMEM;
> -               persistent_ram_write(cxt->cprz, buf, size);
> +               persistent_ram_write(cxt->cprz, buf, size, 1);
>                 return 0;
>         } else if (type == PSTORE_TYPE_FTRACE) {
>                 if (!cxt->fprz)
>                         return -ENOMEM;
> -               persistent_ram_write(cxt->fprz, buf, size);
> +               persistent_ram_write(cxt->fprz, buf, size, 1);
>                 return 0;
>         } else if (type == PSTORE_TYPE_PMSG) {
>                 if (!cxt->mprz)
>                         return -ENOMEM;
> -               persistent_ram_write(cxt->mprz, buf, size);
> +               persistent_ram_write(cxt->mprz, buf, size, 1);
>                 return 0;
>         }
>
> @@ -324,7 +324,7 @@ static int notrace ramoops_pstore_write_buf(enum pstore_type_id type,
>         hlen = ramoops_write_kmsg_hdr(prz, compressed);
>         if (size + hlen > prz->buffer_size)
>                 size = prz->buffer_size - hlen;
> -       persistent_ram_write(prz, buf, size);
> +       persistent_ram_write(prz, buf, size, 1);
>
>         cxt->dump_write_cnt = (cxt->dump_write_cnt + 1) % cxt->max_dump_cnt;
>
> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> index cb92055..c4722f0 100644
> --- a/fs/pstore/ram_core.c
> +++ b/fs/pstore/ram_core.c
> @@ -49,13 +49,15 @@ static inline size_t buffer_start(struct persistent_ram_zone *prz)
>  }
>
>  /* increase and wrap the start pointer, returning the old value */
> -static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
> +static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a,
> +                              int lock)
>  {
>         int old;
>         int new;
>         unsigned long flags;
>
> -       raw_spin_lock_irqsave(&prz->buffer_lock, flags);
> +       if (lock)
> +               raw_spin_lock_irqsave(&prz->buffer_lock, flags);
>
>         old = atomic_read(&prz->buffer->start);
>         new = old + a;
> @@ -63,19 +65,21 @@ static size_t buffer_start_add(struct persistent_ram_zone *prz, size_t a)
>                 new -= prz->buffer_size;
>         atomic_set(&prz->buffer->start, new);
>
> -       raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
> +       if (lock)
> +               raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
>
>         return old;
>  }
>
>  /* increase the size counter until it hits the max size */
> -static void buffer_size_add(struct persistent_ram_zone *prz, size_t a)
> +static void buffer_size_add(struct persistent_ram_zone *prz, size_t a, int lock)
>  {
>         size_t old;
>         size_t new;
>         unsigned long flags;
>
> -       raw_spin_lock_irqsave(&prz->buffer_lock, flags);
> +       if (lock)
> +               raw_spin_lock_irqsave(&prz->buffer_lock, flags);
>
>         old = atomic_read(&prz->buffer->size);
>         if (old == prz->buffer_size)
> @@ -87,7 +91,8 @@ static void buffer_size_add(struct persistent_ram_zone *prz, size_t a)
>         atomic_set(&prz->buffer->size, new);
>
>  exit:
> -       raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
> +       if (lock)
> +               raw_spin_unlock_irqrestore(&prz->buffer_lock, flags);
>  }
>
>  static void notrace persistent_ram_encode_rs8(struct persistent_ram_zone *prz,
> @@ -300,7 +305,7 @@ void persistent_ram_save_old(struct persistent_ram_zone *prz)
>  }
>
>  int notrace persistent_ram_write(struct persistent_ram_zone *prz,
> -       const void *s, unsigned int count)
> +       const void *s, unsigned int count, int lock)
>  {
>         int rem;
>         int c = count;
> @@ -311,9 +316,9 @@ int notrace persistent_ram_write(struct persistent_ram_zone *prz,
>                 c = prz->buffer_size;
>         }
>
> -       buffer_size_add(prz, c);
> +       buffer_size_add(prz, c, lock);
>
> -       start = buffer_start_add(prz, c);
> +       start = buffer_start_add(prz, c, lock);
>
>         rem = prz->buffer_size - start;
>         if (unlikely(rem < c)) {
> @@ -342,9 +347,9 @@ int notrace persistent_ram_write_user(struct persistent_ram_zone *prz,
>                 c = prz->buffer_size;
>         }
>
> -       buffer_size_add(prz, c);
> +       buffer_size_add(prz, c, 1);
>
> -       start = buffer_start_add(prz, c);
> +       start = buffer_start_add(prz, c, 1);
>
>         rem = prz->buffer_size - start;
>         if (unlikely(rem < c)) {
> diff --git a/include/linux/pstore_ram.h b/include/linux/pstore_ram.h
> index 244d242..782af68 100644
> --- a/include/linux/pstore_ram.h
> +++ b/include/linux/pstore_ram.h
> @@ -61,7 +61,7 @@ void persistent_ram_free(struct persistent_ram_zone *prz);
>  void persistent_ram_zap(struct persistent_ram_zone *prz);
>
>  int persistent_ram_write(struct persistent_ram_zone *prz, const void *s,
> -                        unsigned int count);
> +                        unsigned int count, int lock);
>  int persistent_ram_write_user(struct persistent_ram_zone *prz,
>                               const void __user *s, unsigned int count);
>
> --
> 2.7.4
>



-- 
Kees Cook
Nexus Security