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

[Jerin Jacob Kollanukkaran <jerinj@marvell.com>]

> -----Original Message-----
> From: Neil Horman <nhorman@tuxdriver.com>
> Sent: Wednesday, June 5, 2019 11:41 PM
> To: Bruce Richardson <bruce.richardson@intel.com>
> Cc: Jerin Jacob Kollanukkaran <jerinj@marvell.com>; dev@dpdk.org; Thomas
> Monjalon <thomas@monjalon.net>
> Subject: Re: [EXT] [RFC PATCH 0/2] introduce __rte_internal tag
> 
> 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.

I agree too. IMHO, Simply having following items would be enough

1) Avoid exposing the internal function prototype through public header files
2) Add @internal to API documentation
3) Just decide the name space for internal API for tooling(i.e not start with rte_ or so)
Using objdump scheme to detect internal functions requires the the library to build prior to run the checkpatch.

> 
> 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
> >