Re: [PATCH v2] modversions: treat symbol CRCs as 32 bit quantities on 64 bit archs

[Ard Biesheuvel]

On 26 October 2016 at 11:07, Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> Hi Ard,
>
> I like the concept, but ...
>
> Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> writes:
>> The symbol CRCs are emitted as ELF symbols, which allows us to easily
>> populate the kcrctab sections by relying on the linker to associate
>> each kcrctab slot with the correct value.
>>
>> This has two downsides:
>> - given that the CRCs are treated as pointers, we waste 4 bytes for
>>   each CRC on 64 bit architectures,
>> - on architectures that support runtime relocation, a relocation entry is
>>   emitted for each CRC value, which may take up 24 bytes of __init space
>>   (on ELF64 systems)
>>
>> This comes down to a x8 overhead in [uncompressed] kernel size. In addition,
>> each relocation has to be reverted before the CRC value can be used.
>>
>> Switching to explicit 32 bit values on 64 bit architectures fixes both
>> issues, since 32 bit values are not treated as relocatable quantities on
>> ELF64 systems, even if the value ultimately resolves to a linker supplied
>> value.
>
> Are we sure that part is true? ("not treated as relocatable")
>

Thanks for testing this.

> A quick test build on powerpc gives me:
>
>   WARNING: 6829 bad relocations
>   c000000000ca3748 R_PPC64_ADDR16    *ABS*+0x0000000013f53da6
>   c000000000ca374a R_PPC64_ADDR16    *ABS*+0x00000000f7272059
>   c000000000ca374c R_PPC64_ADDR16    *ABS*+0x0000000002013d36
>   c000000000ca374e R_PPC64_ADDR16    *ABS*+0x00000000a59dffc8
>   ...
>
> Which is coming from our relocs_check.sh script, which checks that the
> generated relocations are ones we know how to handle.
>

OK, first of all, it appears the ppc64 assembler interprets .word
differently than the arm64 one, so I will need to fold this in

"""
diff --git a/include/linux/export.h b/include/linux/export.h
index fa51ab2ad190..a000d421526d 100644
--- a/include/linux/export.h
+++ b/include/linux/export.h
@@ -54,7 +54,7 @@ extern struct module __this_module;
 #define __CRC_SYMBOL(sym, sec)                                         \
        asm("   .section \"___kcrctab" sec "+" #sym "\", \"a\"  \n"     \
            "   .weak   " VMLINUX_SYMBOL_STR(__crc_##sym) "     \n"     \
-           "   .word   " VMLINUX_SYMBOL_STR(__crc_##sym) "     \n"     \
+           "   .long   " VMLINUX_SYMBOL_STR(__crc_##sym) "     \n"     \
            "   .previous                                       \n");
 #endif
 #else
"""

With that change, the CRCs are actually emitted as

WARNING: 7525 bad relocations
c000000000ce7f50 R_PPC64_ADDR32    *ABS*+0x0000000013f53da6
c000000000ce7f54 R_PPC64_ADDR32    *ABS*+0x0000000004f7c64e
c000000000ce7f58 R_PPC64_ADDR32    *ABS*+0x0000000092be8a3e

which is still a bit disappointing, given that we still have 7525 RELA
entries to process.
I tried several thing, i.e., adding -Bsymbolic to the linker command
line, emitting the reference above as .hidden or emit  the definition
from the linker script as HIDDEN(), but nothing seems to make any
difference. (On arm64, -Bsymbolic eliminates *all* runtime relocations
except R_<arch>_RELATIVE ones)

> And when I try to boot it I get:
>
>   virtio: disagrees about version of symbol module_layout
>   virtio: disagrees about version of symbol module_layout
>   scsi_mod: disagrees about version of symbol module_layout
>
> And it can't find my root file system (unsurprisingly as it's on scsi).
>

Something like the below should fix it, I hope.

"""
diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S
index d88736fbece6..99cdf2311ab5 100644
--- a/arch/powerpc/kernel/reloc_64.S
+++ b/arch/powerpc/kernel/reloc_64.S
@@ -14,6 +14,7 @@
 RELA = 7
 RELACOUNT = 0x6ffffff9
 R_PPC64_RELATIVE = 22
+R_PPC64_ADDR32 = 1

 /*
  * r3 = desired final address of kernel
@@ -77,9 +78,22 @@ _GLOBAL(relocate)
        add     r0,r0,r3
        stdx    r0,r7,r6
        addi    r9,r9,24
-       bdnz    5b
+       b       7f
+
+       /*
+        * CRCs of exported symbols are emitted as 32-bit relocations against
+        * the *ABS* section with the CRC value recorded in the addend.
+        */
+6:     cmpdi   r0,R_PPC64_ADDR32
+       bne     7f
+       ld      r6,0(r9)        /* reloc->r_offset */
+       ld      r0,16(r9)       /* reloc->r_addend */
+       stwx    r0,r7,r6
+       addi    r9,r9,24
+
+7:     bdnz    5b
+       blr

-6:     blr

 .balign 8
 p_dyn: .llong  __dynamic_start - 0b
"""

Note that the loop no longer terminates at the first
non-R_PPC64_RELATIVE relocation, but that seems safer to me in any
case. It simply stores the value of r_addend at r_offset, which is the
correct thing to do for R_PPC64_ADDR32 relocations against the *ABS*
section, regardless of whether we are dealing with CRCs or something
else. Note that the comparison above will fail for R_PPC64_ADDR32
relocations against named symbols, since we compare the entire r_info
field and not just the type (as the comment a few lines higher up
suggests)

Also a fix for relocs_check.sh:

"""
diff --git a/arch/powerpc/relocs_check.sh b/arch/powerpc/relocs_check.sh
index ec2d5c835170..2f510fbc87da 100755
--- a/arch/powerpc/relocs_check.sh
+++ b/arch/powerpc/relocs_check.sh
@@ -43,7 +43,8 @@ R_PPC_ADDR16_HA
 R_PPC_RELATIVE
 R_PPC_NONE' |
        grep -E -v '\<R_PPC64_ADDR64[[:space:]]+mach_' |
-       grep -E -v '\<R_PPC64_ADDR64[[:space:]]+__crc_'
+       grep -E -v '\<R_PPC64_ADDR64[[:space:]]+__crc_' |
+       grep -E -v '\<R_PPC64_ADDR32[[:space:]]+\*ABS\*'
 )

 if [ -z "$bad_relocs" ]; then
"""

If these changes work for PPC, I think we should fold them in.
Hopefully, GNU ld for PPC will gain that ability to resolve absolute
relocations at build time (like other architectures), and then the
R_PPC64_ADDR32 handling will simply become dead code. In any case, it
is an improvement over the mangling of CRC values to undo the runtime
relocation, imo.

Regards,
Ard.