Re: [PATCH v2 01/16] liveupdate: luo_file: Add internal APIs for file preservation

[Ankit Soni]

On Mon, May 18, 2026 at 07:08:01PM +0000, Samiullah Khawaja wrote:
> On Mon, May 18, 2026 at 11:40:58AM +0000, Pranjal Shrivastava wrote:
> > On Mon, Apr 27, 2026 at 05:56:18PM +0000, Samiullah Khawaja wrote:
> > > From: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > > 
> > > The core liveupdate mechanism allows userspace to preserve file
> > > descriptors. However, kernel subsystems often manage struct file
> > > objects directly and need to participate in the preservation process
> > > programmatically without relying solely on userspace interaction.
> > > 
> > > Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > > Signed-off-by: Samiullah Khawaja <skhawaja@xxxxxxxxxx>
> > 

[..]

> > > + *
> > > + * Context: It must be called with session mutex acquired of a restored session.
> > > + * Return: 0 on success. Returns -ENOENT if no file with the matching token is
> > > + *         found, or any other negative errno on failure.
> > > + */
> > > +int liveupdate_get_file_incoming(struct liveupdate_session *s, u64 token,
> > > +				 struct file **filep)
> > > +{
> > > +	return luo_retrieve_file(luo_file_set_from_session_locked(s),
> > > +				 token, filep);
> > > +}
> > 
> > Nit: Shouldn't we export both of these functions via EXPORT_SYMBOL_GPL?
> > Since, these new APIs are intended for kernel subsystems to participate
> > programmatically, there could be IOMMU drivers (or others) that can be
> > compiled as loadable modules. Thus we should export these APIs via
> > EXPORT_SYMBOL_GPL(). If they aren't exported, any loadable module
> > attempting to use them will compile successfully (due to the header), but
> > will fail to load at runtime with an Unknown symbol error.
> > 
> > IIUC, if a function isn't exported with EXPORT_SYMBOL, it remains hidden
> > inside vmlinux, (i.e. it isn't in the kernel's global symbol table used
> > during modprobe).
> 
> Agreed. Will Update this.
> 
> I think there are other LUO APIs that are not exported and can be used
> by modules. Let me check those also.

Hi,
+1 to Pranjal's EXPORT_SYMBOL_GPL nit, and I'd like to extend it.
With CONFIG_IOMMUFD=m && CONFIG_IOMMU_LIVEUPDATE=y
the series fails at modpost, not at modprobe — kernel won't even build:

  ERROR: modpost: "memfd_get_seals" [iommufd.ko] undefined!
  ERROR: modpost: "iommu_preserve_device" [iommufd.ko] undefined!
  ERROR: modpost: "iommu_unpreserve_device" [iommufd.ko] undefined!
  ERROR: modpost: "liveupdate_register_file_handler" [iommufd.ko] undefined!
  ERROR: modpost: "liveupdate_get_token_outgoing" [iommufd.ko] undefined!
  ERROR: modpost: "iommufd_liveupdate_register" [iommufd.ko] undefined!
  ERROR: modpost: "iommufd_liveupdate_unregister" [iommufd.ko] undefined!
  ERROR: modpost: "iommufd_hwpt_liveupdate_mark_preserve" [iommufd.ko] undefined!

Two issues:
1) drivers/iommu/iommufd/Makefile (patch 12) builds liveupdate.o into vmlinux
   instead of iommufd.ko — needs to be a composite member, not a top-level obj:
     -obj-$(CONFIG_IOMMU_LIVEUPDATE) += liveupdate.o
     +iommufd-$(CONFIG_IOMMU_LIVEUPDATE) += liveupdate.o

   This fixes the iommufd_liveupdate_{register,unregister} and
   iommufd_hwpt_liveupdate_mark_preserve errors above.


2) Beyond the two LUO functions Pranjal mentioned, four more vmlinux symbols
   need EXPORT_SYMBOL_GPL.

Thanks,
Ankit

> > 
> > Thanks,
> > Praan
> > 
> > [1] https://elixir.bootlin.com/linux/v7.0-rc3/source/kernel/liveupdate/luo_file.c#L560
> >