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

From: Kees Cook
Date: Mon Oct 10 2016 - 19:48:39 EST

On Fri, Oct 7, 2016 at 10:28 PM, Joel Fernandes <joelaf@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
> 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