Re: [dpdk-dev] [EXT] [RFC PATCH 0/2] introduce __rte_internal tag

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Jun 05, 2019 at 05:45:41PM +0100, Bruce Richardson wrote:
> On Wed, Jun 05, 2019 at 04:24:09PM +0000, Jerin Jacob Kollanukkaran wrote:
> > > -----Original Message-----
> > > From: Neil Horman <nhorman@tuxdriver.com>
> > > Sent: Sunday, May 26, 2019 12:14 AM
> > > To: dev@dpdk.org
> > > Cc: Neil Horman <nhorman@tuxdriver.com>; Jerin Jacob Kollanukkaran
> > > <jerinj@marvell.com>; Bruce Richardson <bruce.richardson@intel.com>;
> > > Thomas Monjalon <thomas@monjalon.net>
> > > Subject: [EXT] [RFC PATCH 0/2] introduce __rte_internal tag
> > > 
> > > Hey-
> > > 	Based on our recent conversations regarding the use of symbols only
> > > meant for internal dpdk consumption (between dpdk libraries), this is an idea
> > > that I've come up with that I'd like to get some feedback on
> > > 
> > > Summary:
> > > 1) We have symbols in the DPDK that are meant to be used between DPDK
> > > libraries, but not by applications linking to them
> > > 2) We would like to document those symbols in the code, so as to note them
> > > clearly as for being meant for internal use only
> > > 3) Linker symbol visibility is a very coarse grained tool, and so there is no good
> > > way in a single library to mark items as being meant for use only by other
> > > DPDK libraries, at least not without some extensive runtime checking
> > > 
> > > 
> > > Proposal:
> > > I'm proposing that we introduce the __rte_internal tag.  From a coding
> > > standpoint it works a great deal like the __rte_experimental tag in that it
> > > expempts the tagged symbol from ABI constraints (as the only users should
> > > be represented in the DPDK build environment).  Additionally, the
> > > __rte_internal macro resolves differently based on the definition of the
> > > BUILDING_RTE_SDK flag (working under the assumption that said flag should
> > > only ever be set if we are actually building DPDK libraries which will make use
> > > of internal calls).  If the BUILDING_RTE_SDK flag is set __rte_internal resolves
> > > to __attribute__((section "text.internal)), placing it in a special text section
> > > which is then used to validate that the the symbol appears in the INTERNAL
> > > section of the corresponding library version map).  If BUILDING_RTE_SDK is
> > > not set, then __rte_internal resolves to __attribute__((error("..."))), which
> > > causes any caller of the tagged function to throw an error at compile time,
> > > indicating that the symbol is not available for external use.
> > > 
> > > This isn't a perfect solution, as applications can still hack around it of course,
> > 
> > I think, one way to, avoid, hack around could be to,
> > 
> > 1) at config stage, create  a random number for the build
> > 2) introduce RTE_CALL_INTERNAL macro for calling internal function, compare 
> > the generated random number for allowing the calls to make within the library. i.e leverage the
> > fact that external library would never know the random number generated 
> > for the DPDK build and internal driver code does.
> > 
> Do we really need to care about this. If have some determined enough to
> hack around our limitations, then they surely know that they have an
> unsupported configuration. We just need to protect against inadvertent use
> of internals, IMHO.
> 
I agree, I too had thought about doing some sort of internal runtime checking to
match internal only symbols, such that they were only accessable by internally
approved users, but it started to feel like a great deal of overhead.  Its a
good idea for a general mechanism I think, but I believe the value here is more
to internally document which apis we want to mark as being for internal use
only, and create a lightweight roadblock at build time to catch users
inadvertently using them.  Determined users will get around anything, and theres
not much we can do to stop them.

If we really wanted to go down that road, we could use a mechainsm simmilar to
the EXPORT_SYMBOL / EXPORT_SYMBOL_GPL infrastructure that the kernel uses, but
that would required building our own custom linker script, which seems like
overkill here.

Best
Neil

> /Bruce
>