Re: [PATCH v2 03/10] module: add support for symbol namespaces.

[Greg KH]

On Tue, Aug 13, 2019 at 01:17:00PM +0100, Matthias Maennich wrote:
> The EXPORT_SYMBOL_NS() and EXPORT_SYMBOL_NS_GPL() macros can be used to
> export a symbol to a specific namespace.  There are no _GPL_FUTURE and
> _UNUSED variants because these are currently unused, and I'm not sure
> they are necessary.
> 
> I didn't add EXPORT_SYMBOL_NS() for ASM exports; this patch sets the
> namespace of ASM exports to NULL by default. In case of relative
> references, it will be relocatable to NULL. If there's a need, this
> should be pretty easy to add.
> 
> A module that wants to use a symbol exported to a namespace must add a
> MODULE_IMPORT_NS() statement to their module code; otherwise, modpost
> will complain when building the module, and the kernel module loader
> will emit an error and fail when loading the module.
> 
> MODULE_IMPORT_NS() adds a modinfo tag 'import_ns' to the module. That
> tag can be observed by the modinfo command, modpost and kernel/module.c
> at the time of loading the module.
> 
> The ELF symbols are renamed to include the namespace with an asm label;
> for example, symbol 'usb_stor_suspend' in namespace USB_STORAGE becomes
> 'usb_stor_suspend.USB_STORAGE'.  This allows modpost to do namespace
> checking, without having to go through all the effort of parsing ELF and
> relocation records just to get to the struct kernel_symbols.
> 
> On x86_64 I saw no difference in binary size (compression), but at
> runtime this will require a word of memory per export to hold the
> namespace. An alternative could be to store namespaced symbols in their
> own section and use a separate 'struct namespaced_kernel_symbol' for
> that section, at the cost of making the module loader more complex.
> 
> Co-developed-by: Martijn Coenen <maco@xxxxxxxxxxx>
> Signed-off-by: Martijn Coenen <maco@xxxxxxxxxxx>
> Signed-off-by: Matthias Maennich <maennich@xxxxxxxxxx>

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>