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Date:   Wed, 28 Feb 2018 19:49:37 -0500
From:   Gabriel Somlo <somlo@cmu.edu>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     =?iso-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@gmail.com>,
        open list <linux-kernel@vger.kernel.org>, slp@redhat.com,
        bhe@redhat.com, xiaolong.ye@intel.com
Subject: Re: [PATCH v15 08/11] fw_cfg: handle fw_cfg_read_blob() error
Message-ID: <20180301004937.GB25373@crash.ini.cmu.edu>
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On Thu, Mar 01, 2018 at 01:58:22AM +0200, Michael S. Tsirkin wrote:
> On Thu, Mar 01, 2018 at 01:58:01AM +0200, Michael S. Tsirkin wrote:
> > On Wed, Feb 28, 2018 at 06:25:58PM -0500, Gabriel Somlo wrote:
> > > On Wed, Feb 28, 2018 at 05:32:52PM +0200, Michael S. Tsirkin wrote:
> > > > On Wed, Feb 28, 2018 at 12:49:35PM +0100, Marc-Andr? Lureau wrote:
> > > > > Hi
> > > > > 
> > > > > On Tue, Feb 27, 2018 at 1:20 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > > On Thu, Feb 15, 2018 at 10:33:09PM +0100, Marc-Andr? Lureau wrote:
> > > > > >> fw_cfg_read_blob() may fail, but does not return error. This may lead
> > > > > >> to undefined behaviours, such as a memcmp(sig, "QEMU") on uninitilized
> > > > > >> memory.
> > > > > >
> > > > > > I don't think that's true - there's a memset there that
> > > > > > will initialize the memory. probe is likely the only
> > > > > > case where it returns a slightly incorrect data.
> > > > > 
> > > > > Right, I'll update the commit message.
> > > > > 
> > > > > >> Return an error if ACPI locking failed. Also, the following
> > > > > >> DMA read/write extension will add more error paths that should be
> > > > > >> handled appropriately.
> > > > > >>
> > > > > >> Signed-off-by: Marc-Andr? Lureau <marcandre.lureau@redhat.com>
> > > > > >> ---
> > > > > >>  drivers/firmware/qemu_fw_cfg.c | 32 ++++++++++++++++++++++----------
> > > > > >>  1 file changed, 22 insertions(+), 10 deletions(-)
> > > > > >>
> > > > > >> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> > > > > >> index f6f90bef604c..5e6e5ac71dab 100644
> > > > > >> --- a/drivers/firmware/qemu_fw_cfg.c
> > > > > >> +++ b/drivers/firmware/qemu_fw_cfg.c
> > > > > >> @@ -59,8 +59,8 @@ static void fw_cfg_sel_endianness(u16 key)
> > > > > >>  }
> > > > > >>
> > > > > >>  /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
> > > > > >> -static void fw_cfg_read_blob(u16 key,
> > > > > >> -                     void *buf, loff_t pos, size_t count)
> > > > > >> +static ssize_t fw_cfg_read_blob(u16 key,
> > > > > >> +                             void *buf, loff_t pos, size_t count)
> > > > > >>  {
> > > > > >>       u32 glk = -1U;
> > > > > >>       acpi_status status;
> > > > > >> @@ -73,7 +73,7 @@ static void fw_cfg_read_blob(u16 key,
> > > > > >>               /* Should never get here */
> > > > > >>               WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
> > > > > >>               memset(buf, 0, count);
> > > > > >> -             return;
> > > > > >> +             return -EINVAL;
> > > > > >>       }
> > > > > >>
> > > > > >>       mutex_lock(&fw_cfg_dev_lock);
> > > > > >
> > > > > > Wouldn't something like -EBUSY be more appropriate?
> > > > > 
> > > > > In theory, it would be a general failure right? I don't think we want
> > > > > the caller to retry. I think in EINVAL fits better, but I don't think
> > > > > it matters much this or EBUSY.
> > > > > 
> > > > > >> @@ -84,6 +84,7 @@ static void fw_cfg_read_blob(u16 key,
> > > > > >>       mutex_unlock(&fw_cfg_dev_lock);
> > > > > >>
> > > > > >>       acpi_release_global_lock(glk);
> > > > > >> +     return count;
> > > > > >>  }
> > > > > >>
> > > > > >>  /* clean up fw_cfg device i/o */
> > > > > >> @@ -165,8 +166,9 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
> > > > > >>       }
> > > > > >>
> > > > > >>       /* verify fw_cfg device signature */
> > > > > >> -     fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
> > > > > >> -     if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
> > > > > >> +     if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
> > > > > >> +                             0, FW_CFG_SIG_SIZE) < 0 ||
> > > > > >> +             memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
> > > > > >>               fw_cfg_io_cleanup();
> > > > > >>               return -ENODEV;
> > > > > >>       }
> > > > > >> @@ -326,8 +328,7 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
> > > > > >>       if (count > entry->size - pos)
> > > > > >>               count = entry->size - pos;
> > > > > >>
> > > > > >> -     fw_cfg_read_blob(entry->select, buf, pos, count);
> > > > > >> -     return count;
> > > > > >> +     return fw_cfg_read_blob(entry->select, buf, pos, count);
> > > > > >>  }
> > > > > >>
> > > > > >>  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> > > > > >> @@ -483,7 +484,11 @@ static int fw_cfg_register_dir_entries(void)
> > > > > >>       struct fw_cfg_file *dir;
> > > > > >>       size_t dir_size;
> > > > > >>
> > > > > >> -     fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count, 0, sizeof(files_count));
> > > > > >> +     ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count,
> > > > > >> +                     0, sizeof(files_count));
> > > > > >> +     if (ret < 0)
> > > > > >> +             return ret;
> > > > > >> +
> > > > > >>       count = be32_to_cpu(files_count);
> > > > > >>       dir_size = count * sizeof(struct fw_cfg_file);
> > > > > >>
> > > > > >> @@ -491,7 +496,10 @@ static int fw_cfg_register_dir_entries(void)
> > > > > >>       if (!dir)
> > > > > >>               return -ENOMEM;
> > > > > >>
> > > > > >> -     fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files_count), dir_size);
> > > > > >> +     ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
> > > > > >> +                     sizeof(files_count), dir_size);
> > > > > >> +     if (ret < 0)
> > > > > >> +             goto end;
> > > > > >>
> > > > > >>       for (i = 0; i < count; i++) {
> > > > > >>               ret = fw_cfg_register_file(&dir[i]);
> > > > > >> @@ -499,6 +507,7 @@ static int fw_cfg_register_dir_entries(void)
> > > > > >>                       break;
> > > > > >>       }
> > > > > >>
> > > > > >> +end:
> > > > > >>       kfree(dir);
> > > > > >>       return ret;
> > > > > >>  }
> > > > > >> @@ -539,7 +548,10 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
> > > > > >>               goto err_probe;
> > > > > >>
> > > > > >>       /* get revision number, add matching top-level attribute */
> > > > > >> -     fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
> > > > > >> +     err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
> > > > > >> +     if (err < 0)
> > > > > >> +             goto err_probe;
> > > > > >> +
> > > > > >>       fw_cfg_rev = le32_to_cpu(rev);
> > > > > >>       err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
> > > > > >>       if (err)
> > > > > >
> > > > > > I think that this is the only case where it's not doing the right thing right now in
> > > > > > that it shows 0 as the revision to the users.  Is it worth failing probe
> > > > > > here?  We could just skip the attribute, could we not?
> > > > > 
> > > > > I think it's best to fail the probe if we have a read failure at that time.
> > > > 
> > > > I'd rather we just dropped this attribute completely.
> > > > Why is it there?
> > > > Does any userspace actually need it?
> > > > Gabriel?
> > > 
> > > I'd recommend keeping it: `cat /sys/firmware/qemu_fw_cfg/rev` is how
> > > you can easily tell if, e.g., dma is supported :)
> 
> Does user ever care?

I'd rather not presume they *don't*

For instance, yourself: you add DMA support, then one day in the
future on some system you're on, things feel sluggish. You're curious
-- does this thing claim to support DMA? The answer is one easy `cat`
away. If you then found out someone decided you're not supposed to care,
and forced you to dig through the sources for a guess instead, you'd
probably be less than happy about it... :)

That said, if there's a better reason to leave it out than handling
some never-to-occur error condition elegantly, I can be persuaded...

As I replied to Michael earlier, leaving it out if that impossible
error condition *does* occur is OK with me (since it "never happens"
in practice... :)

Thanks,
--G

> > > If you end up with a '0' there, it's because locking ACPI with
> > > WAIT_FOREVER failed with something other than AE_NOT_CONFIGURED, which
> > > should never happen, so we're off the reservation, having morally
> > > failed the equivalent of an assertion.
> > > 
> > > Perhaps memset to something other than 0 (all-Fs, so we'd get a -1 for
> > > rev)? 
> > > 
> > > Thanks,
> > > --Gabriel
> > 
> > Why not just skip adding the attribute on this error?
> > 
> > > > 
> > > > > -- 
> > > > > Marc-Andr? Lureau