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SubjectRe: [PATCH 1/2] livepatch: Initialize shadow variables by init function safely
On Tue, Mar 13, 2018 at 04:54:47PM +0100, Petr Mladek wrote:
> The existing API allows to pass a sample data to initialize the shadow
> data. It works well when the data are position independent. But it fails
> miserably when we need to set a pointer to the shadow structure itself.
>
> Unfortunately, we might need to initialize the pointer surprisingly
> often because of struct list_head. It is even worse because the list
> might be hidden in other common structures, for example, struct mutex,
> struct wait_queue_head.
>
> This patch makes the API more safe. A custom init function and data
> are passed to klp_shadow_*alloc() functions instead of the sample data.

Yup, this looks kinda familiar, I remember tinkering with the same idea
last year [1] before settling on the simpler API.

[1] https://github.com/torvalds/linux/compare/master...joe-lawrence:shadow_variables_v2_c

> Note that the init_data are not longer a template for the shadow->data.
> It might point to any data that might be necessary when the init
> function is called.

I'm not opposed to changing the API, but I was wondering if you had
thought about expanding it as an alternative?

When working on this last summer, I remember holding onto to some less
than intuitive naming conventions so that I could support a basic API
and an extended API with bells and whistles like this patchset
implements. It didn't seem too difficult to layer the basic API ontop
of one like this (see [1] for example), so maybe that's an option to
keep basic shadow variable usage a little simpler. /two cents

> In addition, the newly allocated shadow structure is initialized
> only when it is really used. For this, the init function must be
> called under klp_shadow_lock. On one hand, this adds a risk of
> ABBA deadlocks. On the other hand, it allows to do some operations
> safely. For example, we could add the new structure into an
> existing list.
>
> Reported-by: Nicolai Stange <nstange@suse.de>
> Signed-off-by: Petr Mladek <pmladek@suse.com>
> ---
> Documentation/livepatch/shadow-vars.txt | 32 +++++++++++++++------
> include/linux/livepatch.h | 17 ++++++++---
> kernel/livepatch/shadow.c | 48 +++++++++++++++++++++----------
> samples/livepatch/livepatch-shadow-fix1.c | 19 +++++++++++-
> samples/livepatch/livepatch-shadow-fix2.c | 6 ++--
> 5 files changed, 89 insertions(+), 33 deletions(-)
>
> diff --git a/Documentation/livepatch/shadow-vars.txt b/Documentation/livepatch/shadow-vars.txt
> [ ... snip ...]
> @@ -148,16 +154,24 @@ shadow variables to parents already in-flight.
> For commit 1d147bfa6429, a good spot to allocate a shadow spinlock is
> inside ieee80211_sta_ps_deliver_wakeup():
>
> +int ps_lock_shadow_init(void *obj, void *shadow_data, void *data)
> +{
> + spinlock_t *lock = shadow_data;
> +
> + spin_lock_init(lock);
> + return 0;
> +}
> +
> #define PS_LOCK 1
> void ieee80211_sta_ps_deliver_wakeup(struct sta_info *sta)
> {
> - DEFINE_SPINLOCK(ps_lock_fallback);
> spinlock_t *ps_lock;
>
> /* sync with ieee80211_tx_h_unicast_ps_buf */
> ps_lock = klp_shadow_get_or_alloc(sta, PS_LOCK,
> - &ps_lock_fallback, sizeof(ps_lock_fallback),
> - GFP_ATOMIC);
> + sizeof(ps_lock_fallback), GFP_ATOMIC,

I think this should be "sizeof(*ps_lock)" here since we've removed the
ps_lock_fallback.

> + ps_lock_shadow_init, NULL);
> +
> if (ps_lock)
> spin_lock(ps_lock);

The rest of this patchset looks pretty good. I gave the samples a
test-run and they still operate as advertised. Perhaps shadow variables
are another candidate for some kind of kselftest?

Regards,

-- Joe

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