linux.git/tools/objtool/elf.c, branch v5.18-rc2

objtool: Ignore extra-symbol code

2022-03-15T09:32:43+00:00

There's a fun implementation detail on linking STB_WEAK symbols. When
the linker combines two translation units, where one contains a weak
function and the other an override for it. It simply strips the
STB_WEAK symbol from the symbol table, but doesn't actually remove the
code.

The result is that when objtool is ran in a whole-archive kind of way,
it will encounter *heaps* of unused (and unreferenced) code. All
rudiments of weak functions.

Additionally, when a weak implementation is split into a .cold
subfunction that .cold symbol is left in place, even though completely
unused.

Teach objtool to ignore such rudiments by searching for symbol holes;
that is, code ranges that fall outside the given symbol bounds.
Specifically, ignore a sequence of unreachable instruction iff they
occupy a single hole, additionally ignore any .cold subfunctions
referenced.

Both ld.bfd and ld.lld behave like this. LTO builds otoh can (and do)
properly DCE weak functions.

Signed-off-by: Peter Zijlstra (Intel) 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154319.232019347@infradead.org

objtool: Add --dry-run

2022-03-15T09:32:32+00:00

Add a --dry-run argument to skip writing the modifications. This is
convenient for debugging.

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Kees Cook 
Reviewed-by: Miroslav Benes 
Acked-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20220308154317.282720146@infradead.org

objtool: Fix pv_ops noinstr validation

2021-12-03T08:11:42+00:00

Boris reported that in one of his randconfig builds, objtool got
infinitely stuck. Turns out there's trivial list corruption in the
pv_ops tracking when a function is both in a static table and in a code
assignment.

Avoid re-adding function to the pv_ops[] lists when they're already on
it.

Fixes: db2b0c5d7b6f ("objtool: Support pv_opsindirect calls for noinstr")
Reported-by: Borislav Petkov 
Signed-off-by: Peter Zijlstra (Intel) 
Signed-off-by: Borislav Petkov 
Tested-by: Borislav Petkov 
Link: https://lkml.kernel.org/r/20211202204534.GA16608@worktop.programming.kicks-ass.net

Merge tag 'objtool-core-2021-10-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2021-11-01T20:24:43+00:00

Pull objtool updates from Thomas Gleixner:

 - Improve retpoline code patching by separating it from alternatives
   which reduces memory footprint and allows to do better optimizations
   in the actual runtime patching.

 - Add proper retpoline support for x86/BPF

 - Address noinstr warnings in x86/kvm, lockdep and paravirtualization
   code

 - Add support to handle pv_opsindirect calls in the noinstr analysis

 - Classify symbols upfront and cache the result to avoid redundant
   str*cmp() invocations.

 - Add a CFI hash to reduce memory consumption which also reduces
   runtime on a allyesconfig by ~50%

 - Adjust XEN code to make objtool handling more robust and as a side
   effect to prevent text fragmentation due to placement of the
   hypercall page.

* tag 'objtool-core-2021-10-31' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (41 commits)
  bpf,x86: Respect X86_FEATURE_RETPOLINE*
  bpf,x86: Simplify computing label offsets
  x86,bugs: Unconditionally allow spectre_v2=retpoline,amd
  x86/alternative: Add debug prints to apply_retpolines()
  x86/alternative: Try inline spectre_v2=retpoline,amd
  x86/alternative: Handle Jcc __x86_indirect_thunk_\reg
  x86/alternative: Implement .retpoline_sites support
  x86/retpoline: Create a retpoline thunk array
  x86/retpoline: Move the retpoline thunk declarations to nospec-branch.h
  x86/asm: Fixup odd GEN-for-each-reg.h usage
  x86/asm: Fix register order
  x86/retpoline: Remove unused replacement symbols
  objtool,x86: Replace alternatives with .retpoline_sites
  objtool: Shrink struct instruction
  objtool: Explicitly avoid self modifying code in .altinstr_replacement
  objtool: Classify symbols
  objtool: Support pv_opsindirect calls for noinstr
  x86/xen: Rework the xen_{cpu,irq,mmu}_opsarrays
  x86/xen: Mark xen_force_evtchn_callback() noinstr
  x86/xen: Make irq_disable() noinstr
  ...

objtool,x86: Replace alternatives with .retpoline_sites

2021-10-28T21:25:25+00:00

Instead of writing complete alternatives, simply provide a list of all
the retpoline thunk calls. Then the kernel is free to do with them as
it pleases. Simpler code all-round.

Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: Borislav Petkov 
Acked-by: Josh Poimboeuf 
Tested-by: Alexei Starovoitov 
Link: https://lore.kernel.org/r/20211026120309.850007165@infradead.org

objtool: Update section header before relocations

2021-10-07T03:11:57+00:00

The libelf implementation from elftoolchain has a safety check in
gelf_update_rel[a] to check that the data corresponds to a section
that has type SHT_REL[A] [0]. If the relocation is updated before
the section header is updated with the proper type, this check
fails.

To fix this, update the section header first, before the relocations.
Previously, the section size was calculated in elf_rebuild_reloc_section
by counting the number of entries in the reloc_list. However, we
now need the size during elf_write so instead keep a running total
and add to it for every new relocation.

[0] https://sourceforge.net/p/elftoolchain/mailman/elftoolchain-developers/thread/CAGw6cBtkZro-8wZMD2ULkwJ39J+tHtTtAWXufMjnd3cQ7XG54g@mail.gmail.com/

Signed-off-by: Michael Forney 
Reviewed-by: Miroslav Benes 
Signed-off-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20210509000103.11008-2-mforney@mforney.org

objtool: Check for gelf_update_rel[a] failures

2021-10-07T03:11:53+00:00

Otherwise, if these fail we end up with garbage data in the
.rela.orc_unwind_ip section, leading to errors like

  ld: fs/squashfs/namei.o: bad reloc symbol index (0x7f16 >= 0x12) for offset 0x7f16d5c82cc8 in section `.orc_unwind_ip'

Signed-off-by: Michael Forney 
Reviewed-by: Miroslav Benes 
Signed-off-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20210509000103.11008-1-mforney@mforney.org

objtool: Remove redundant 'len' field from struct section

2021-10-05T19:03:21+00:00

The section structure already contains sh_size, so just remove the extra
'len' member that requires extra mirroring and potential confusion.

Suggested-by: Josh Poimboeuf 
Signed-off-by: Joe Lawrence 
Reviewed-by: Miroslav Benes 
Signed-off-by: Josh Poimboeuf 
Link: https://lore.kernel.org/r/20210822225037.54620-3-joe.lawrence@redhat.com
Cc: Andy Lavr 
Cc: Peter Zijlstra 
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org

Merge tags 'objtool-urgent-2021-06-28' and 'objtool-core-2021-06-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

2021-06-28T18:35:55+00:00

Pull objtool fix and updates from Ingo Molnar:
 "An ELF format fix for a section flags mismatch bug that breaks kernel
  tooling such as kpatch-build.

  The biggest change in this cycle is the new code to handle and rewrite
  variable sized jump labels - which results in slightly tighter code
  generation in hot paths, through the use of short(er) NOPs.

  Also a number of cleanups and fixes, and a change to the generic
  include/linux/compiler.h to handle a s390 GCC quirk"

* tag 'objtool-urgent-2021-06-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  objtool: Don't make .altinstructions writable

* tag 'objtool-core-2021-06-28' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  objtool: Improve reloc hash size guestimate
  instrumentation.h: Avoid using inline asm operand modifiers
  compiler.h: Avoid using inline asm operand modifiers
  kbuild: Fix objtool dependency for 'OBJECT_FILES_NON_STANDARD_ := n'
  objtool: Reflow handle_jump_alt()
  jump_label/x86: Remove unused JUMP_LABEL_NOP_SIZE
  jump_label, x86: Allow short NOPs
  objtool: Provide stats for jump_labels
  objtool: Rewrite jump_label instructions
  objtool: Decode jump_entry::key addend
  jump_label, x86: Emit short JMP
  jump_label: Free jump_entry::key bit1 for build use
  jump_label, x86: Add variable length patching support
  jump_label, x86: Introduce jump_entry_size()
  jump_label, x86: Improve error when we fail expected text
  jump_label, x86: Factor out the __jump_table generation
  jump_label, x86: Strip ASM jump_label support
  x86, objtool: Dont exclude arch/x86/realmode/
  objtool: Rewrite hashtable sizing

objtool: Improve reloc hash size guestimate

2021-06-14T12:05:36+00:00

Nathan reported that LLVM ThinLTO builds have a performance regression
with commit 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing"). Sami
was quick to note that this is due to their use of -ffunction-sections.

As a result the .text section is small and basing the number of relocs
off of that no longer works. Instead have read_sections() compute the
sum of all SHF_EXECINSTR sections and use that.

Fixes: 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing")
Reported-by: Nathan Chancellor 
Debugged-by: Sami Tolvanen 
Signed-off-by: Peter Zijlstra (Intel) 
Tested-by: Nathan Chancellor 
Link: https://lkml.kernel.org/r/YMJpGLuGNsGtA5JJ@hirez.programming.kicks-ass.net