[PATCH kernel v2] powerpc/boot: Stop using RELACOUNT

[Alexey Kardashevskiy]

On 4/6/22 14:58, Gabriel Paubert wrote:
> On Wed, Apr 06, 2022 at 02:01:48PM +1000, Alexey Kardashevskiy wrote:
>> So far the RELACOUNT tag from the ELF header was containing the exact
>> number of R_PPC_RELATIVE/R_PPC64_RELATIVE relocations. However the LLVM's
>> recent change [1] make it equal-or-less than the actual number which
>> makes it useless.
>>
>> This replaces RELACOUNT in zImage loader with a pair of RELASZ and RELAENT.
>> The vmlinux relocation code is fixed in commit d79976918852
>> ("powerpc/64: Add UADDR64 relocation support").
>>
>> To make it more future proof, this walks through the entire .rela.dyn
>> section instead of assuming that the section is sorter by a relocation
>> type. Unlike d79976918852, this does not add unaligned UADDR/UADDR64
>> relocations as we are likely not to see those in practice - the zImage
>> is small and very arch specific so there is a smaller chance that some
>> generic feature (such as PRINK_INDEX) triggers unaligned relocations.
>>
>> [1] https://github.com/llvm/llvm-project/commit/da0e5b885b25cf4
>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>> ---
>> Changes:
>> v2:
>> * s/divd/divwu/ for ppc32
>> * updated the commit log
>> * named all new labels instead of numbering them
>> (s/101f/.Lcheck_for_relaent/ and so on)
>> ---
>>   arch/powerpc/boot/crt0.S | 45 ++++++++++++++++++++++++++--------------
>>   1 file changed, 29 insertions(+), 16 deletions(-)
>>
>> diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
>> index feadee18e271..e9306d862f8d 100644
>> --- a/arch/powerpc/boot/crt0.S
>> +++ b/arch/powerpc/boot/crt0.S
>> @@ -8,7 +8,8 @@
>>   #include "ppc_asm.h"
>>   
>>   RELA = 7
>> -RELACOUNT = 0x6ffffff9
>> +RELASZ = 8
>> +RELAENT = 9
>>   
>>   	.data
>>   	/* A procedure descriptor used when booting this as a COFF file.
>> @@ -75,34 +76,39 @@ p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
>>   	bne	11f
>>   	lwz	r9,4(r12)	/* get RELA pointer in r9 */
>>   	b	12f
>> -11:	addis	r8,r8,(-RELACOUNT)@ha
>> -	cmpwi	r8,RELACOUNT at l
>> +11:	cmpwi	r8,RELASZ
>> +	bne	.Lcheck_for_relaent
>> +	lwz	r0,4(r12)       /* get RELASZ value in r0 */
>> +	b	12f
>> +.Lcheck_for_relaent:
>> +	cmpwi	r8,RELAENT
>>   	bne	12f
>> -	lwz	r0,4(r12)	/* get RELACOUNT value in r0 */
>> +	lwz     r14,4(r12)      /* get RELAENT value in r14 */
>>   12:	addi	r12,r12,8
>>   	b	9b
>>   
>>   	/* The relocation section contains a list of relocations.
>>   	 * We now do the R_PPC_RELATIVE ones, which point to words
>> -	 * which need to be initialized with addend + offset.
>> -	 * The R_PPC_RELATIVE ones come first and there are RELACOUNT
>> -	 * of them. */
>> +	 * which need to be initialized with addend + offset */
>>   10:	/* skip relocation if we don't have both */
>>   	cmpwi	r0,0
>>   	beq	3f
>>   	cmpwi	r9,0
>>   	beq	3f
>> +	cmpwi	r14,0
>> +	beq	3f
>>   
>>   	add	r9,r9,r11	/* Relocate RELA pointer */
>> +	divwu   r0,r0,r14       /* RELASZ / RELAENT */
>>   	mtctr	r0
>>   2:	lbz	r0,4+3(r9)	/* ELF32_R_INFO(reloc->r_info) */
>>   	cmpwi	r0,22		/* R_PPC_RELATIVE */
>> -	bne	3f
>> +	bne	.Lnext
>>   	lwz	r12,0(r9)	/* reloc->r_offset */
>>   	lwz	r0,8(r9)	/* reloc->r_addend */
>>   	add	r0,r0,r11
>>   	stwx	r0,r11,r12
>> -	addi	r9,r9,12
>> +.Lnext:	add	r9,r9,r14
>>   	bdnz	2b
>>   
>>   	/* Do a cache flush for our text, in case the loader didn't */
>> @@ -160,32 +166,39 @@ p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
>>   	bne	10f
>>   	ld	r13,8(r11)       /* get RELA pointer in r13 */
>>   	b	11f
>> -10:	addis	r12,r12,(-RELACOUNT)@ha
>> -	cmpdi	r12,RELACOUNT at l
>> -	bne	11f
>> -	ld	r8,8(r11)       /* get RELACOUNT value in r8 */
>> +10:	cmpwi   r12,RELASZ
>> +	bne	.Lcheck_for_relaent
>> +	lwz	r8,8(r11)	/* get RELASZ pointer in r8 */
>> +	b	11f
>> +.Lcheck_for_relaent:
>> +	cmpwi	r12,RELAENT
>> +	bne     11f
>> +	lwz     r14,8(r11)      /* get RELAENT pointer in r14 */
>>   11:	addi	r11,r11,16
>>   	b	9b
>>   12:
>> -	cmpdi	r13,0            /* check we have both RELA and RELACOUNT */
>> +	cmpdi	r13,0            /* check we have both RELA, RELASZ, RELAENT*/
>>   	cmpdi	cr1,r8,0
>>   	beq	3f
>>   	beq	cr1,3f
>> +	cmpdi	r14,0
>> +	beq	3f
>>   
>>   	/* Calcuate the runtime offset. */
>>   	subf	r13,r13,r9
>>   
>>   	/* Run through the list of relocations and process the
>>   	 * R_PPC64_RELATIVE ones. */
>> +	divd    r8,r8,r14       /* RELASZ / RELAENT */
> 
> While you are at it, this one should also be divdu.
> 
> I really wished IBM had used explicit signed/unsigned indication in the
> mnemonics (divds, divdu, divws, divwu) instead. Fortunately very little
> assemby code uses these instructions nowadays.


Fair enough, v3 is coming. Thanks,

> 
> 
>>   	mtctr	r8
>>   13:	ld	r0,8(r9)        /* ELF64_R_TYPE(reloc->r_info) */
>>   	cmpdi	r0,22           /* R_PPC64_RELATIVE */
>> -	bne	3f
>> +	bne	.Lnext
>>   	ld	r12,0(r9)        /* reloc->r_offset */
>>   	ld	r0,16(r9)       /* reloc->r_addend */
>>   	add	r0,r0,r13
>>   	stdx	r0,r13,r12
>> -	addi	r9,r9,24
>> +.Lnext:	add	r9,r9,r14
>>   	bdnz	13b
>>   
>>   	/* Do a cache flush for our text, in case the loader didn't */
>> -- 
>> 2.30.2
> 
> 	Cheers,
> 	Gabriel
>   
>