Messages in this thread | | | Date | Wed, 1 Apr 2020 14:08:55 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH 3/5] objtool: Support Clang non-section symbols in ORC generation |
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On Wed, Apr 01, 2020 at 02:05:51PM -0500, Josh Poimboeuf wrote: > On Wed, Apr 01, 2020 at 08:49:53PM +0200, Peter Zijlstra wrote: > > On Wed, Apr 01, 2020 at 01:23:27PM -0500, Josh Poimboeuf wrote: > > > > > @@ -105,8 +100,32 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti > > > } > > > memset(rela, 0, sizeof(*rela)); > > > > > > - rela->sym = insn_sec->sym; > > > - rela->addend = insn_off; > > > + if (insn_sec->sym) { > > > + rela->sym = insn_sec->sym; > > > + rela->addend = insn_off; > > > + } else { > > > + /* > > > + * The Clang assembler doesn't produce section symbols, so we > > > + * have to reference the function symbol instead: > > > + */ > > > + rela->sym = find_symbol_containing(insn_sec, insn_off); > > > > It's a good thing I made that a lot faster I suppose ;-) > > :-) > > > > + if (!rela->sym) { > > > + /* > > > + * Hack alert. This happens when we need to reference > > > + * the NOP pad insn immediately after the function. > > > + */ > > > + rela->sym = find_symbol_containing(insn_sec, > > > + insn_off - 1); > > > > Urgh, when does that happen? > > It happens naturally in the padding between functions, since objtool > doesn't traverse those instructions. So they have undefined entries > like > > .text+68: sp:(und) bp:(und) type:call end:0 > > I suppose those aren't technically necessary.
In fact, we could probably get substantial savings in the ORC table if we skipped those, i.e.
.text+0: sp:sp+8 bp:(und) type:call end:0 .text+8: sp:(und) bp:(und) type:call end:0 .text+10: sp:sp+8 bp:(und) type:call end:0 .text+17: sp:sp+16 bp:(und) type:call end:0 .text+18: sp:sp+24 bp:prevsp-24 type:call end:0 .text+1c: sp:sp+32 bp:prevsp-24 type:call end:0 .text+5a: sp:sp+24 bp:prevsp-24 type:call end:0 .text+61: sp:sp+16 bp:(und) type:call end:0 .text+63: sp:sp+8 bp:(und) type:call end:0 .text+68: sp:(und) bp:(und) type:call end:0 .text+70: sp:sp+8 bp:(und) type:call end:0 .text+8c: sp:(und) bp:(und) type:call end:0 .text+90: sp:sp+8 bp:(und) type:call end:0 .text+cd: sp:(und) bp:(und) type:call end:0 .text+d0: sp:sp+8 bp:(und) type:call end:0
would be compressed to
.text+0: sp:sp+8 bp:(und) type:call end:0 .text+17: sp:sp+16 bp:(und) type:call end:0 .text+18: sp:sp+24 bp:prevsp-24 type:call end:0 .text+1c: sp:sp+32 bp:prevsp-24 type:call end:0 .text+5a: sp:sp+24 bp:prevsp-24 type:call end:0 .text+61: sp:sp+16 bp:(und) type:call end:0 .text+63: sp:sp+8 bp:(und) type:call end:0
but I can do that in a separate patch, and if it works I can remove this hack.
-- Josh
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