linux-kernel - Re: [PATCH v2] pstore: Avoid duplicate call of persistent_ram

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Date:   Mon, 29 Oct 2018 21:22:08 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peng15 Wang 王鹏 <wangpeng15@...omi.com>
Cc:     Kees Cook <keescook@...omium.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] pstore: Avoid duplicate call of persistent_ram_zap()

On Mon, Oct 29, 2018 at 06:37:53AM +0000, Peng15 Wang 王鹏 wrote:
> 
> ________________________________________
> >From: Kees Cook <keescook@...omium.org>
> >Sent: Monday, October 29, 2018 0:03
> >To: Peng15 Wang 王鹏
> >Cc: anton@...msg.org; ccross@...roid.com; tony.luck@...el.com; linux-kernel@...r.kernel.org; Joel Fernandes
> >Subject: Re: [PATCH v2] pstore: Avoid duplicate call of persistent_ram_zap()
> >
> >On Sat, Oct 27, 2018 at 2:08 PM, Peng15 Wang 王鹏 <wangpeng15@...omi.com> wrote:
> >> When initialing prz with invalid data in buffer(no PERSISTENT_RAM_SIG),
> >> function call path is like this:
> >>
> >> ramoops_init_prz ->
> >> |
> >> |-> persistent_ram_new -> persistent_ram_post_init -> persistent_ram_zap
> >> |
> >> |-> persistent_ram_zap
> >>
> >> As we can see, persistent_ram_zap() is called twice.
> >> We can avoid this by adding an option to persistent_ram_new(), and
> >> only call persistent_ram_zap() when it is needed.
> >>
> >> Signed-off-by: Peng Wang <wangpeng15@...omi.com>
> >> ---
> >>  fs/pstore/ram.c            |  5 +++--
> >>  fs/pstore/ram_core.c       | 11 +++++++----
> >>  include/linux/pstore_ram.h |  3 ++-
> >>  3 files changed, 12 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> >> index ffcff6516e89..3044274de2f0 100644
> >> --- a/fs/pstore/ram.c
> >> +++ b/fs/pstore/ram.c
> >> @@ -596,7 +596,8 @@ static int ramoops_init_przs(const char *name,
> >>                                           name, i, *cnt - 1);
> >>                 prz_ar[i] = persistent_ram_new(*paddr, zone_sz, sig,
> >>                                                &cxt->ecc_info,
> >> -                                              cxt->memtype, flags, label);
> >> +                                              cxt->memtype, flags,
> >> +                                              label, true);
> >>                 if (IS_ERR(prz_ar[i])) {
> >>                         err = PTR_ERR(prz_ar[i]);
> >>                         dev_err(dev, "failed to request %s mem region (0x%zx@...llx): %d\n",
> >> @@ -640,7 +641,7 @@ static int ramoops_init_prz(const char *name,
> >>
> >>         label = kasprintf(GFP_KERNEL, "ramoops:%s", name);
> >>         *prz = persistent_ram_new(*paddr, sz, sig, &cxt->ecc_info,
> >> -                                 cxt->memtype, 0, label);
> >> +                                 cxt->memtype, 0, label, false);
> >>         if (IS_ERR(*prz)) {
> >>                 int err = PTR_ERR(*prz);
> >>
> >> diff --git a/fs/pstore/ram_core.c b/fs/pstore/ram_core.c
> >> index 12e21f789194..d8a520c8741c 100644
> >> --- a/fs/pstore/ram_core.c
> >> +++ b/fs/pstore/ram_core.c
> >> @@ -486,7 +486,8 @@ static int persistent_ram_buffer_map(phys_addr_t start, phys_addr_t size,
> >>  }
> >>
> >>  static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 sig,
> >> -                                   struct persistent_ram_ecc_info *ecc_info)
> >> +                                   struct persistent_ram_ecc_info *ecc_info,
> >> +                                   bool zap_option)
> >>  {
> >>         int ret;
> >>
> >> @@ -514,7 +515,8 @@ static int persistent_ram_post_init(struct persistent_ram_zone *prz, u32 >sig,
> >>
> >>         /* Rewind missing or invalid memory area. */
> >>         prz->buffer->sig = sig;
> >> -       persistent_ram_zap(prz);
> >> +       if (zap_option)
> >> +               persistent_ram_zap(prz);
> >
> >This part of persistent_ram_post_init() handles the "invalid buffer"
> >case, which should always zap. The question is whether or not to zap
> >in the case of a valid buffer (the "return 0" earlier in the
> >function). I think you v2 patch needs similar changes found in your
> >v1: the v2 patch also needs to remove the "return 0" and replace it
> >with "zap_option = true;" and to remove the zap call from
> >ramoops_init_prz(). Then I think all the paths will be consolidated.
> 
> Thank you so much for the tips!
> 
> Furthermore,  we can make "zap_option" stand for whether its caller want to zap in case of
> a valid buffer. So ramoops_init_przs() would say "false", and ramoops_init_prz() would 
> say "true".
> 
> In persistent_ram_post_init(), if zap_option says "false", we return immediately after 
> persistent_ram_save_old(), otherwise persistent_ram_zap would be called at the end.

Can you not just add it to the flags, something like PRZ_ZAP_NEW, and set
that flag before calling ramoops_init_prz*, then check the flag in
persistent_ram_new? We are already passing flags to persistent_ram_new.

That way no new function arguments are needed and its simple.

 - Joel