Re: [PATCH] module: Fix link failure due to invalid relocation on namespace offset

From: Matthias Maennich
Date: Wed Sep 11 2019 - 12:53:50 EST

On Wed, Sep 11, 2019 at 05:40:13PM +0100, Will Deacon wrote:
On Wed, Sep 11, 2019 at 03:35:06PM +0200, Jessica Yu wrote:
+++ Will Deacon [11/09/19 13:26 +0100]:
> Commit 8651ec01daed ("module: add support for symbol namespaces.")
> broke linking for arm64 defconfig:
> | lib/crypto/arc4.o: In function `__ksymtab_arc4_setkey':
> | arc4.c:(___ksymtab+arc4_setkey+0x8): undefined reference to `no symbol'
> | lib/crypto/arc4.o: In function `__ksymtab_arc4_crypt':
> | arc4.c:(___ksymtab+arc4_crypt+0x8): undefined reference to `no symbol'
> This is because the dummy initialisation of the 'namespace_offset' field
> in 'struct kernel_symbol' when using EXPORT_SYMBOL on architectures with
> support for PREL32 locations uses an offset from an absolute address (0)
> in an effort to trick 'offset_to_pointer' into behaving as a NOP,
> allowing non-namespaced symbols to be treated in the same way as those
> belonging to a namespace.
> Unfortunately, place-relative relocations require a symbol reference
> rather than an absolute value and, although x86 appears to get away with
> this due to placing the kernel text at the top of the address space, it
> almost certainly results in a runtime failure if the kernel is relocated
> dynamically as a result of KASLR.
> Rework 'namespace_offset' so that a value of 0, which cannot occur for a
> valid namespaced symbol, indicates that the corresponding symbol does
> not belong to a namespace.
> Cc: Matthias Maennich <maennich@xxxxxxxxxx>
> Cc: Jessica Yu <jeyu@xxxxxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Fixes: 8651ec01daed ("module: add support for symbol namespaces.")
> Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>
> ---
> Please note that I've not been able to test this at LPC, but it's been
> submitted to kernelci.

Thanks for fixing this so quickly. I can confirm that this fixes the
build for arm64 defconfig and x86 built fine for me as well. I'll wait
a bit and apply this at the end of the day in case Matthias or anybody
else would like to confirm/test.

FWIW, I've managed to boot arm64 Debian under QEMU and load/unload
modules successfully with this patch applied on top of modules-next.

Thanks Will for fixing this so quickly! The patch looks good to me.

Feel free to add
Reviewed-by: Matthias Maennich <maennich@xxxxxxxxxx>
Tested-by: Matthias Maennich <maennich@xxxxxxxxxx>