Re: [PATCH net-next v2 2/2] net: ipa: fix IPA validation

On Sun, Mar 21, 2021 at 12:19:02PM -0500, Alex Elder wrote:

On 3/21/21 8:49 AM, Leon Romanovsky wrote:

quoted

On Sun, Mar 21, 2021 at 08:21:24AM -0500, Alex Elder wrote:

quoted

On 3/21/21 3:21 AM, Leon Romanovsky wrote:

quoted

On Sat, Mar 20, 2021 at 09:17:29AM -0500, Alex Elder wrote:

quoted

There are blocks of IPA code that sanity-check various values, at
compile time where possible.  Most of these checks can be done once
during development but skipped for normal operation.  These checks
permit the driver to make certain assumptions, thereby avoiding the
need for runtime error checking.

The checks are defined conditionally, but not consistently.  In
some cases IPA_VALIDATION enables the optional checks, while in
others IPA_VALIDATE is used.

Fix this by using IPA_VALIDATION consistently.

Signed-off-by: Alex Elder <redacted>
---
  drivers/net/ipa/Makefile       | 2 +-
  drivers/net/ipa/gsi_trans.c    | 8 ++++----
  drivers/net/ipa/ipa_cmd.c      | 4 ++--
  drivers/net/ipa/ipa_cmd.h      | 6 +++---
  drivers/net/ipa/ipa_endpoint.c | 6 +++---
  drivers/net/ipa/ipa_main.c     | 6 +++---
  drivers/net/ipa/ipa_mem.c      | 6 +++---
  drivers/net/ipa/ipa_table.c    | 6 +++---
  drivers/net/ipa/ipa_table.h    | 6 +++---
  9 files changed, 25 insertions(+), 25 deletions(-)

diff --git a/drivers/net/ipa/Makefile b/drivers/net/ipa/Makefile
index afe5df1e6eeee..014ae36ac6004 100644
--- a/drivers/net/ipa/Makefile
+++ b/drivers/net/ipa/Makefile

@@ -1,5 +1,5 @@
  # Un-comment the next line if you want to validate configuration data
-#ccflags-y		+=	-DIPA_VALIDATE
+# ccflags-y		+=	-DIPA_VALIDATION

Maybe netdev folks think differently here, but general rule that dead
code and closed code is such, is not acceptable to in Linux kernel.

<...>

What is the purpose of CONFIG_KGDB?  Or CONFIG_DEBUG_KERNEL?
Would you prefer I expose this through a kconfig option?  I
intentionally did not do that, because I really intended it
to be only for development, so defined it in the Makefile.
But I have no objection to making it configurable that way.

I prefer you to follow netdev/linux kernel rules of development.
The upstream repository and drivers/net/* folder especially are not
the place to put code used for the development.

How do I add support for new versions of the hardware as
it evolves?

Exactly like all other driver developers do. You are not different here.

1. Clean your driver to have stable base without dead/debug code.
2. Send patch series per-feature/hardware enablement on top of this base.

What I started supporting (v3.5.1) was in some respects
relatively old.  Version 4.2 is newer, and the v4.5 and
beyond are for products that are relatively new on the
market.

I see that it was submitted in 2018, we have many large drivers
that by far older than IPA. For example, mlx5 supports more than 5
generations of hardware and was added in 2013.

Some updates to IPA (like 4.0+ after 3.5.1, or 4.5+
after 4.2) include substantial updates to the way the
hardware works.  The code can't support the new hardware
without being adapted and generalized to support both
old and new.

It is ok.

My goal is to get upstream support for IPA for all
Qualcomm SoCs that have it.  But the hardware design
is evolving; Qualcomm is actively developing their
architecture so they can support new technologies
(e.g. cellular 5G).  Development of the driver is
simply *necessary*.

No argue here.

The assertions I proposed and checks like this are
intended as an *aid* to the active development I
have been doing.

They need to be local to your development environment.
It is perfectly fine if you keep extra debug patch internally.

They may look like hacky debugging--checking errors
that can't happen.  They aren't that at all--they're
intended to the compiler help me develop correct code,
given I *know* it will be evolving.

It is wrong assumption that you are alone who are reading this code.
I presented my view as a casual developer who sometimes need to change
code that is not my expertise.

The extra checks, unreachable and/or for non-existing code are very similar
to the bad comments - they make simple tasks too complex, it causes to us
wonder why they exist, maybe I broke something, e.t.c.

Unreachable code and checks sometimes serve as a hint for code deletion
and this is exactly how I spotted your driver.

But the assertions are gone, and I accept/agree that
these specific checks "look funny."  More below.

quoted

quoted

quoted

quoted

-#ifdef IPA_VALIDATE
+#ifdef IPA_VALIDATION
  	if (!size || size % 8)
  		return -EINVAL;
  	if (count < max_alloc)
  		return -EINVAL;
  	if (!max_alloc)
  		return -EINVAL;
-#endif /* IPA_VALIDATE */
+#endif /* IPA_VALIDATION */

If it is possible to supply those values, the check should be always and
not only under some closed config option.

These are assertions.

There is no need to test them for working code.  If
I run the code successfully with these tests enabled
exactly once, and they are satisfied, then every time
the code is run thereafter they will pass.  So I want
to check them when debugging/developing only.  That
way there is a mistake, it gets caught, but otherwise
there's no pointless argument checking done.

Like I said below, you are not alone who are reading this code.
The kernel has three ways to handle wrong flow:
1. if ... return; -- used for runtime checks of possible but wrong input, or userspace input
2. BUILD_BUG_ON() -- used for compilation checks
3. WARN_ON*()     -- used for runtime checks of not-possible kernel flows

You are trying to use something different which is not needed.

quoted

quoted

I'll explain the first check; the others have similar
explanation.

In the current code, the passed size is sizeof(struct)
for three separate structures.
  - If the structure size changes, I want to be
    sure the constraint is still honored
  - The code will break of someone happens
    to pass a size of 0.  I don't expect that to
    ever happen, but this states that requirement.

This is an optimization, basically, but one that
allows the assumed conditions to be optionally
verified.

Everything above as an outcome of attempting to mix constant vs. run-time
checks. If "size" is constant, the use of BUILD_BIG_ON() will help not only
you but other developers to catch the errors too. The assumption that you alone
are working on this code, can or can't be correct.

Right now I am the only one doing substantive development.
I am listed as the maintainer, and I trust anything more
than simple fixes will await my review before being
merged.

It is wrong assumption too. Major changes are performed all the time and
not always maintainers have say, sometimes it is because of timing, sometimes
it is because right thing to do.

quoted

If "size" is not constant, you should check it always.

To do that I might need to make this function (and others
like it) inline, or maybe __always_inline.  Regardless,
I generally agree with your suggestion of defensively
testing the argument value.  But this is an *internal
interface*.  The only callers are inside the driver.

This is checked with WARN_ON*().

It's basically putting the burden on the caller to verify
parameters, because often the caller already knows.

I think this is more of a philosophical argument than
a technical one.  The check isn't *that* expensive.

My complain is lack of readability and maintainability for random kernel
developers and not performance concerns. 

quoted

quoted

quoted

quoted

  	/* By allocating a few extra entries in our pool (one less
  	 * than the maximum number that will be requested in a

@@ -140,14 +140,14 @@ int gsi_trans_pool_init_dma(struct device *dev, struct gsi_trans_pool *pool,
  	dma_addr_t addr;
  	void *virt;
-#ifdef IPA_VALIDATE
+#ifdef IPA_VALIDATION
  	if (!size || size % 8)
  		return -EINVAL;
  	if (count < max_alloc)
  		return -EINVAL;
  	if (!max_alloc)
  		return -EINVAL;
-#endif /* IPA_VALIDATE */
+#endif /* IPA_VALIDATION */

Same

<...>

quoted

  {
-#ifdef IPA_VALIDATE
+#ifdef IPA_VALIDATION
  	/* At one time we assumed a 64-bit build, allowing some do_div()
  	 * calls to be replaced by simple division or modulo operations.
  	 * We currently only perform divide and modulo operations on u32,

@@ -768,7 +768,7 @@ static void ipa_validate_build(void)
  	BUILD_BUG_ON(!ipa_aggr_granularity_val(IPA_AGGR_GRANULARITY));
  	BUILD_BUG_ON(ipa_aggr_granularity_val(IPA_AGGR_GRANULARITY) >
  			field_max(AGGR_GRANULARITY_FMASK));
-#endif /* IPA_VALIDATE */
+#endif /* IPA_VALIDATION */

BUILD_BUG_ON()s are checked during compilation and not during runtime
like IPA_VALIDATION promised.

So I should update the description.  But I'm not sure where
you are referring to.  Here is the first line of the patch
description:
  There are blocks of IPA code that sanity-check various
  values, at compile time where possible.

I'm suggesting to review if IPA_VALIDATION is truly needed.

*That* is a suggestion I can act on...

Right now, it's *there*.  These few patches were the beginning
of a side task to simplify and/or get rid of it.  The first
step is to get it so it's not fundamentally broken.  Then I
can work on getting rid of (or at least refactor) pieces.

The code I started with did lots of checks of these things
(including build-time checkable ones).  Many, many functions
needlessly returned values, just so these checks could be made.
The possibility of returning an error meant all callers had
to check for it, and that complicated things all the way up.

So I tried to gather such things into foo_validate() functions,
which just grouped these checks without having to clutter the
normal code path with them.  That way called functions could
have void return type, and calling functions would be simpler,
and so on.

So I guess to respond again to your comment, I really would
like to get rid of IPA_VALIDATION, or most of it.  As it is,
many things are checked with BUILD_BUG_ON(), but they need
not really be conditionally built.  That is a fix I intend
to make, but haven't yet.

But the code is there, and if I am going to fix it, I need
to do it with patches.  And I try to make my patches small
enough to be easily reviewable.

quoted

quoted

quoted

IMHO, the issue here is that this IPA code isn't release quality but
some debug drop variant and it is far from expected from submitted code.

Doesn't sound very humble, IMHO.

Sorry about that.

I'd like to suggest a plan so I can begin to make progress,
but do so in a way you/others think is satisfactory.
- I would first like to fix the existing bugs, namely that
  if IPA_VALIDATION is defined there are build errors, and
  that IPA_VALIDATION is not consistently used.  That is
  this 2-patch series.

The thing is that IPA_VALIDATION is not defined in the upstream kernel.
There is nothing to be fixed in netdev repository

- I assure you that my goal is to simplify the code that
  does this sort of checking.  So here are some specific
  things I can implement in the coming weeks toward that:
    - Anything that can be checked at build time, will
      be checked with BUILD_BUG_ON().

+1

    - Anything checked with BUILD_BUG_ON() will *not*
      be conditional.  I.e. it won't be inside an
      #ifdef IPA_VALIDATION block.
    - I will review all remaining VALIDATION code (which
      can't--or can't always--be checked at build time),
      If it looks prudent to make it *always* be checked,
      I will make it always be checked (not conditional
      on IPA_VALIDATION).

+1

The result should clearly separate checks that can be done
at build time from those that can't.

And with what's left (especially on that third sub-bullet)
I might have some better examples with which to argue
for something different.  Or I might just concede that
you were right all along.

I hope so.

Thanks