On Tue, Mar 08, 2022 at 09:51:18PM -0800, Fangrui Song wrote: > DT_RELACOUNT is an ELF dynamic tag inherited from SunOS indicating the > number of R_*_RELATIVE relocations. It is optional but {ld.lld,ld.lld}
^ ld.{bfd,lld} ? > -z combreloc always creates it (if non-zero) to slightly speed up glibc > ld.so relocation resolving by avoiding R_*R_PPC64_RELATIVE type > comparison. The tag is otherwise nearly unused in the wild and I'd > recommend that software avoids using it. > > lld>=14.0.0 (since commit da0e5b885b25cf4ded0fa89b965dc6979ac02ca9) > underestimates DT_RELACOUNT for ppc64 when position-independent long > branch thunks are used. Correcting it needs non-trivial arch-specific > complexity which I'd prefer to avoid. Since our code always compares the > relocation type with R_PPC64_RELATIVE, replacing every occurrence of > DT_RELACOUNT with DT_RELASZ/sizeof(Elf64_Rela)=DT_RELASZ/24 is a correct > alternative. > > DT_RELASZ is in practice bounded by an uint32_t. Dividing x by 24 can be > implemented as (uint32_t)(x*0xaaaaaaab) >> 4. > > Link: https://github.com/ClangBuiltLinux/linux/issues/1581 > Reported-by: Nathan Chancellor <nat...@kernel.org> > Signed-off-by: Fangrui Song <mask...@google.com> Thank you a lot for the fix! I tested this with several different configurations with tip of tree LLVM and they all booted in QEMU. I also tested one configuration with GCC 5.4 (we support GCC 5.x as a minimum) and it booted as well. Tested-by: Nathan Chancellor <nat...@kernel.org> > --- > arch/powerpc/boot/crt0.S | 28 +++++++++++++++++----------- > arch/powerpc/kernel/reloc_64.S | 15 +++++++++------ > 2 files changed, 26 insertions(+), 17 deletions(-) > > diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S > index feadee18e271..1c96ebe7ef1a 100644 > --- a/arch/powerpc/boot/crt0.S > +++ b/arch/powerpc/boot/crt0.S > @@ -8,7 +8,7 @@ > #include "ppc_asm.h" > > RELA = 7 > -RELACOUNT = 0x6ffffff9 > +RELASZ = 8 > > .data > /* A procedure descriptor used when booting this as a COFF file. > @@ -65,7 +65,7 @@ p_base: mflr r10 /* r10 now points to > runtime addr of p_base */ > subf r11,r11,r12 /* runtime - linktime offset */ > > /* The dynamic section contains a series of tagged entries. > - * We need the RELA and RELACOUNT entries. */ > + * We need the RELA and RELASZ entries. */ > li r9,0 > li r0,0 > 9: lwz r8,0(r12) /* get tag */ > @@ -75,18 +75,21 @@ 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 12f > - lwz r0,4(r12) /* get RELACOUNT value in r0 */ > + lwz r0,4(r12) /* get RELASZ / 24 in r0 */ > + lis r8,0xaaaa > + ori r8,r8,0xaaab > + mulhwu r0,r0,r8 > + srwi r0,r0,4 > 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. */ > + * The R_PPC_RELATIVE ones come first and there are at most > + * RELASZ/24 of them. */ > 10: /* skip relocation if we don't have both */ > cmpwi r0,0 > beq 3f > @@ -160,14 +163,17 @@ 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@l > +10: cmpdi r12,RELASZ > bne 11f > - ld r8,8(r11) /* get RELACOUNT value in r8 */ > + ld r8,8(r11) /* get RELASZ / 24 in r8 */ > + lis r0,0xaaaa > + ori r0,r0,0xaaab > + mulhwu r8,r8,r0 > + srwi r8,r8,4 > 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 and RELASZ */ > cmpdi cr1,r8,0 > beq 3f > beq cr1,3f > diff --git a/arch/powerpc/kernel/reloc_64.S b/arch/powerpc/kernel/reloc_64.S > index 02d4719bf43a..362be759609f 100644 > --- a/arch/powerpc/kernel/reloc_64.S > +++ b/arch/powerpc/kernel/reloc_64.S > @@ -8,7 +8,7 @@ > #include <asm/ppc_asm.h> > > RELA = 7 > -RELACOUNT = 0x6ffffff9 > +RELASZ = 8 > R_PPC64_RELATIVE = 22 > > /* > @@ -27,7 +27,7 @@ _GLOBAL(relocate) > add r10,r10,r12 /* r10 has runtime addr of _stext */ > > /* > - * Scan the dynamic section for the RELA and RELACOUNT entries. > + * Scan the dynamic section for the RELA and RELASZ entries. > */ > li r7,0 > li r8,0 > @@ -38,13 +38,16 @@ _GLOBAL(relocate) > bne 2f > ld r7,8(r11) /* get RELA pointer in r7 */ > b 3f > -2: addis r6,r6,(-RELACOUNT)@ha > - cmpdi r6,RELACOUNT@l > +2: cmpdi r6,RELASZ > bne 3f > - ld r8,8(r11) /* get RELACOUNT value in r8 */ > + ld r8,8(r11) /* get RELA / 24 in r8 */ > + lis r0,0xaaaa > + ori r0,r0,0xaaab > + mulhwu r8,r8,r0 > + srwi r8,r8,4 > 3: addi r11,r11,16 > b 1b > -4: cmpdi r7,0 /* check we have both RELA and RELACOUNT */ > +4: cmpdi r7,0 /* check we have both RELA and RELASZ */ > cmpdi cr1,r8,0 > beq 6f > beq cr1,6f > -- > 2.35.1.616.g0bdcbb4464-goog >