EXPORT_SYMBOL and __init is a bad combination because the .init.text
section is freed up after the initialization. Hence, modules cannot
use symbols annotated __init. The access to a freed symbol may end up
with kernel panic.

modpost used to detect it, but it has been broken for a decade.

Recently, I fixed modpost so it started to warn it again, then this
showed up in linux-next builds.

There are two ways to fix it:

  - Remove __init
  - Remove EXPORT_SYMBOL

I chose the latter for this case because none of the in-tree call-sites
(arch/arm/xen/enlighten.c, arch/x86/xen/grant-table.c) is compiled as
modular.

Fixes: 243848fc018c ("xen/grant-table: Move xlated_setup_gnttab_pages to common place")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

 drivers/xen/xlate_mmu.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
index 34742c6e189e..f17c4c03db30 100644
--- a/drivers/xen/xlate_mmu.c
+++ b/drivers/xen/xlate_mmu.c
@@ -261,7 +261,6 @@ int __init xen_xlate_map_ballooned_pages(xen_pfn_t **gfns, void **virt,
 
 	return 0;
 }
-EXPORT_SYMBOL_GPL(xen_xlate_map_ballooned_pages);
 
 struct remap_pfn {
 	struct mm_struct *mm;
-- 
2.32.0

On 06.06.22 06:59, Masahiro Yamada wrote:
> EXPORT_SYMBOL and __init is a bad combination because the .init.text
> section is freed up after the initialization. Hence, modules cannot
> use symbols annotated __init. The access to a freed symbol may end up
> with kernel panic.
> 
> modpost used to detect it, but it has been broken for a decade.
> 
> Recently, I fixed modpost so it started to warn it again, then this
> showed up in linux-next builds.
> 
> There are two ways to fix it:
> 
>    - Remove __init
>    - Remove EXPORT_SYMBOL
> 
> I chose the latter for this case because none of the in-tree call-sites
> (arch/arm/xen/enlighten.c, arch/x86/xen/grant-table.c) is compiled as
> modular.
> 
> Fixes: 243848fc018c ("xen/grant-table: Move xlated_setup_gnttab_pages to common place")
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Pushed to xen/tip.git for-linus-5.19a


Juergen

On 06.06.22 07:59, Masahiro Yamada wrote:

Hello

> EXPORT_SYMBOL and __init is a bad combination because the .init.text
> section is freed up after the initialization. Hence, modules cannot
> use symbols annotated __init. The access to a freed symbol may end up
> with kernel panic.
>
> modpost used to detect it, but it has been broken for a decade.
>
> Recently, I fixed modpost so it started to warn it again, then this
> showed up in linux-next builds.
>
> There are two ways to fix it:
>
>    - Remove __init
>    - Remove EXPORT_SYMBOL
>
> I chose the latter for this case because none of the in-tree call-sites
> (arch/arm/xen/enlighten.c, arch/x86/xen/grant-table.c) is compiled as
> modular.

Good description.


>
> Fixes: 243848fc018c ("xen/grant-table: Move xlated_setup_gnttab_pages to common place")
> Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

I think the patch is correct.

Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

> ---
>
>   drivers/xen/xlate_mmu.c | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
> index 34742c6e189e..f17c4c03db30 100644
> --- a/drivers/xen/xlate_mmu.c
> +++ b/drivers/xen/xlate_mmu.c
> @@ -261,7 +261,6 @@ int __init xen_xlate_map_ballooned_pages(xen_pfn_t **gfns, void **virt,
>   
>   	return 0;
>   }
> -EXPORT_SYMBOL_GPL(xen_xlate_map_ballooned_pages);
>   
>   struct remap_pfn {
>   	struct mm_struct *mm;

-- 
Regards,

Oleksandr Tyshchenko

On Mon, 6 Jun 2022, Oleksandr wrote:
> On 06.06.22 07:59, Masahiro Yamada wrote:
> 
> Hello
> 
> > EXPORT_SYMBOL and __init is a bad combination because the .init.text
> > section is freed up after the initialization. Hence, modules cannot
> > use symbols annotated __init. The access to a freed symbol may end up
> > with kernel panic.
> > 
> > modpost used to detect it, but it has been broken for a decade.
> > 
> > Recently, I fixed modpost so it started to warn it again, then this
> > showed up in linux-next builds.
> > 
> > There are two ways to fix it:
> > 
> >    - Remove __init
> >    - Remove EXPORT_SYMBOL
> > 
> > I chose the latter for this case because none of the in-tree call-sites
> > (arch/arm/xen/enlighten.c, arch/x86/xen/grant-table.c) is compiled as
> > modular.
> 
> Good description.
> 
> 
> > 
> > Fixes: 243848fc018c ("xen/grant-table: Move xlated_setup_gnttab_pages to
> > common place")
> > Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
> > Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> 
> I think the patch is correct.
> 
> Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>

Acked-by: Stefano Stabellini <sstabellini@kernel.org>


> > ---
> > 
> >   drivers/xen/xlate_mmu.c | 1 -
> >   1 file changed, 1 deletion(-)
> > 
> > diff --git a/drivers/xen/xlate_mmu.c b/drivers/xen/xlate_mmu.c
> > index 34742c6e189e..f17c4c03db30 100644
> > --- a/drivers/xen/xlate_mmu.c
> > +++ b/drivers/xen/xlate_mmu.c
> > @@ -261,7 +261,6 @@ int __init xen_xlate_map_ballooned_pages(xen_pfn_t
> > **gfns, void **virt,
> >     	return 0;
> >   }
> > -EXPORT_SYMBOL_GPL(xen_xlate_map_ballooned_pages);
> >     struct remap_pfn {
> >   	struct mm_struct *mm;
> 
> -- 
> Regards,
> 
> Oleksandr Tyshchenko
>