On Wed, Mar 23, 2022 at 11:18:40AM +1100, Alexey Kardashevskiy wrote:

On 3/22/22 13:12, Michael Ellerman wrote:

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Alexey Kardashevskiy [off-list ref] writes:

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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 [2].

That's committed so you can say:
   in commit d79976918852 ("powerpc/64: Add UADDR64 relocation support")

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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 [1], this does not add unaligned UADDR/UADDR64 relocations

                 ^
                 that should be 2?

Yes.

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as in hardly possible to see those in arch-specific zImage.

I don't quite parse that. Is it true we can never see them in zImage?
Maybe it's true that we don't see them in practice.

I can force UADDR64 in zImage as I did for d79976918852 but zImage is lot
smaller and more arch-specific than vmlinux and so far only PRINT_INDEX
triggered UADDR64 in vmlinux and chances of the same thing happening in
zImage are small.

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[1] https://github.com/llvm/llvm-project/commit/da0e5b885b25cf4
[2] https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?h=next&id=d799769188529a
Signed-off-by: Alexey Kardashevskiy <redacted>
---
  arch/powerpc/boot/crt0.S | 43 +++++++++++++++++++++++++---------------
  1 file changed, 27 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
index feadee18e271..6ea3417da3b7 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 +76,38 @@ 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@l
+11:	cmpwi	r8,RELASZ
+	bne	111f
+	lwz	r0,4(r12)       /* get RELASZ value in r0 */
+	b	12f
+111:	cmpwi	r8,RELAENT

Can you use named local labels for new labels you introduce?

This could be .Lcheck_for_relaent: perhaps.

Then I'll need to rename them all/most and add more noise to the patch which
reduces chances of it being reviewed. But sure, I can rename labels.

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  	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 */
+	divd    r0,r0,r14       /* RELASZ / RELAENT */

This is in the 32-bit portion isn't it. AFAIK 32-bit CPUs don't
implement divd. I'm not sure why the toolchain allowed it. I would
expect it to trap if run on real 32-bit hardware.

Uff, my bad, "divw", right?

Or maybe even "divwu", since I suspect both values are unsigned.

Probably does not make a difference in practice.

	Gabriel

I am guessing it works as zImage for 64bit BigEndian is still ELF32 which
runs in 64bit CPU and I did not test on real PPC32 as I'm not quite sure how
and I hoped your farm will do this for me :)

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  	mtctr	r0
  2:	lbz	r0,4+3(r9)	/* ELF32_R_INFO(reloc->r_info) */
  	cmpwi	r0,22		/* R_PPC_RELATIVE */
-	bne	3f
+	bne	22f
  	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
+22:	add	r9,r9,r14
  	bdnz	2b
  	/* Do a cache flush for our text, in case the loader didn't */

cheers