linux-toradex.git/tools/objtool/elf.c, branch v6.19-rc2

objtool: Remove second pass of .cold function correlation

2025-11-21T09:04:08+00:00

The .cold function parent/child correlation logic has two passes: one in
read_symbols() and one in add_jump_destinations().

The second pass was added with commit cd77849a69cf ("objtool: Fix GCC 8
cold subfunction detection for aliased functions") to ensure that if the
parent symbol had aliases then the canonical symbol was chosen as the
parent.

That solution was rather clunky, not to mention incomplete due to the
existence of alternatives and switch tables.  Now that we have
sym->alias, the canonical alias fix can be done much simpler in the
first pass, making the second pass obsolete.

Signed-off-by: Josh Poimboeuf 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/bdab245a38000a5407f663a031f39e14c67a43d4.1763671318.git.jpoimboe@kernel.org

objtool: Return canonical symbol when aliases exist in symbol finding helpers

2025-11-21T09:04:08+00:00

When symbol alias ambiguity exists in the symbol finding helper
functions, return the canonical sym->alias, as that's the one which gets
used by validate_branch() and elsewhere.

This doesn't fix any known issues, just makes the symbol alias behavior
more robust.

Signed-off-by: Josh Poimboeuf 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/450470a4897706af77453ad333e18af5ebab653c.1763671318.git.jpoimboe@kernel.org

objtool: Don't alias undefined symbols

2025-11-21T09:04:08+00:00

Objtool is mistakenly aliasing all undefined symbols.  That's obviously
wrong, though it has no consequence since objtool happens to only use
sym->alias for defined symbols.  Fix it regardless.

Signed-off-by: Josh Poimboeuf 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/bc401173a7717757eee672fc1ca5a20451d77b86.1763671318.git.jpoimboe@kernel.org

objtool: Fix .cold function detection for duplicate symbols

2025-11-21T09:04:07+00:00

The objtool .cold child/parent correlation is done in two phases: first
in elf_add_symbol() and later in add_jump_destinations().

The first phase is rather crude and can pick the wrong parent if there
are duplicates with the same name.

The second phase usually fixes that, but only if the parent has a direct
jump to the child.  It does *not* work if the only branch from the
parent to the child is an alternative or jump table entry.

Make the first phase more robust by looking for the parent in the same
STT_FILE as the child.

Fixes the following objtool warnings in an AutoFDO build with a large
CLANG_AUTOFDO_PROFILE profile:

  vmlinux.o: warning: objtool: rdev_add_key() falls through to next function rdev_add_key.cold()
  vmlinux.o: warning: objtool: rdev_set_default_key() falls through to next function rdev_set_default_key.cold()

Fixes: 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions")
Signed-off-by: Josh Poimboeuf 
Signed-off-by: Peter Zijlstra (Intel) 
Link: https://patch.msgid.link/82c7b52e40efa75dd10e1c550cc75c1ce10ac2c9.1763671318.git.jpoimboe@kernel.org

objtool: Support Clang AUTOFDO .cold functions

2025-11-21T09:04:07+00:00

AutoFDO enables -fsplit-machine-functions which can move the cold parts
of a function to a .cold symbol in a .text.split. section.

Unlike GCC, the Clang .cold symbols are not marked STT_FUNC.  This
confuses objtool in several ways, resulting in warnings like the
following:

  vmlinux.o: warning: objtool: apply_retpolines.cold+0xfc: unsupported instruction in callable function
  vmlinux.o: warning: objtool: machine_check_poll.cold+0x2e: unsupported instruction in callable function
  vmlinux.o: warning: objtool: free_deferred_objects.cold+0x1f: relocation to !ENDBR: free_deferred_objects.cold+0x26
  vmlinux.o: warning: objtool: rpm_idle.cold+0xe0: relocation to !ENDBR: rpm_idle.cold+0xe7
  vmlinux.o: warning: objtool: tcp_rcv_state_process.cold+0x1c: relocation to !ENDBR: tcp_rcv_state_process.cold+0x23

Fix it by marking the .cold symbols as STT_FUNC.

Fixes: 2fd65f7afd5a ("AutoFDO: Enable machine function split optimization for AutoFDO")
Closes: https://lore.kernel.org/20251103215244.2080638-2-xur@google.com
Reported-by: Rong Xu 
Signed-off-by: Josh Poimboeuf 
Signed-off-by: Peter Zijlstra (Intel) 
Reviewed-by: xur@google.com
Tested-by: xur@google.com
Link: https://patch.msgid.link/20a67326f04b2a361c031b56d58e8a803b3c5893.1763671318.git.jpoimboe@kernel.org

objtool: Remove unneeded semicolon

2025-10-30T15:29:46+00:00

Remove unnecessary semicolons reported by Coccinelle/coccicheck and the
semantic patch at scripts/coccinelle/misc/semicolon.cocci.

Signed-off-by: Chen Ni 
Link: https://patch.msgid.link/20251020020916.1070369-1-nichen@iscas.ac.cn
Signed-off-by: Josh Poimboeuf

objtool: Add base objtool support for livepatch modules

2025-10-14T21:50:18+00:00

In preparation for klp-build, enable "classic" objtool to work on
livepatch modules:

  - Avoid duplicate symbol/section warnings for prefix symbols and the
    .static_call_sites and __mcount_loc sections which may have already
    been extracted by klp diff.

  - Add __klp_funcs to the IBT function pointer section whitelist.

  - Prevent KLP symbols from getting incorrectly classified as cold
    subfunctions.

Acked-by: Petr Mladek 
Tested-by: Joe Lawrence 
Signed-off-by: Josh Poimboeuf

objtool: Refactor prefix symbol creation code

2025-10-14T21:50:18+00:00

The prefix symbol creation code currently ignores all errors, presumably
because some functions don't have the leading NOPs.

Shuffle the code around a bit, improve the error handling and document
why some errors are ignored.

Acked-by: Petr Mladek 
Tested-by: Joe Lawrence 
Signed-off-by: Josh Poimboeuf

objtool/klp: Introduce klp diff subcommand for diffing object files

2025-10-14T21:50:18+00:00

Add a new klp diff subcommand which performs a binary diff between two
object files and extracts changed functions into a new object which can
then be linked into a livepatch module.

This builds on concepts from the longstanding out-of-tree kpatch [1]
project which began in 2012 and has been used for many years to generate
livepatch modules for production kernels.  However, this is a complete
rewrite which incorporates hard-earned lessons from 12+ years of
maintaining kpatch.

Key improvements compared to kpatch-build:

  - Integrated with objtool: Leverages objtool's existing control-flow
    graph analysis to help detect changed functions.

  - Works on vmlinux.o: Supports late-linked objects, making it
    compatible with LTO, IBT, and similar.

  - Simplified code base: ~3k fewer lines of code.

  - Upstream: No more out-of-tree #ifdef hacks, far less cruft.

  - Cleaner internals: Vastly simplified logic for symbol/section/reloc
    inclusion and special section extraction.

  - Robust __LINE__ macro handling: Avoids false positive binary diffs
    caused by the __LINE__ macro by introducing a fix-patch-lines script
    (coming in a later patch) which injects #line directives into the
    source .patch to preserve the original line numbers at compile time.

Note the end result of this subcommand is not yet functionally complete.
Livepatch needs some ELF magic which linkers don't like:

  - Two relocation sections (.rela*, .klp.rela*) for the same text
    section.

  - Use of SHN_LIVEPATCH to mark livepatch symbols.

Unfortunately linkers tend to mangle such things.  To work around that,
klp diff generates a linker-compliant intermediate binary which encodes
the relevant KLP section/reloc/symbol metadata.

After module linking, a klp post-link step (coming soon) will clean up
the mess and convert the linked .ko into a fully compliant livepatch
module.

Note this subcommand requires the diffed binaries to have been compiled
with -ffunction-sections and -fdata-sections, and processed with
'objtool --checksum'.  Those constraints will be handled by a klp-build
script introduced in a later patch.

Without '-ffunction-sections -fdata-sections', reliable object diffing
would be infeasible due to toolchain limitations:

  - For intra-file+intra-section references, the compiler might
    occasionally generated hard-coded instruction offsets instead of
    relocations.

  - Section-symbol-based references can be ambiguous:

    - Overlapping or zero-length symbols create ambiguity as to which
      symbol is being referenced.

    - A reference to the end of a symbol (e.g., checking array bounds)
      can be misinterpreted as a reference to the next symbol, or vice
      versa.

A potential future alternative to '-ffunction-sections -fdata-sections'
would be to introduce a toolchain option that forces symbol-based
(non-section) relocations.

Acked-by: Petr Mladek 
Tested-by: Joe Lawrence 
Signed-off-by: Josh Poimboeuf

objtool/klp: Add --checksum option to generate per-function checksums

2025-10-14T21:50:17+00:00

In preparation for the objtool klp diff subcommand, add a command-line
option to generate a unique checksum for each function.  This will
enable detection of functions which have changed between two versions of
an object file.

Acked-by: Petr Mladek 
Tested-by: Joe Lawrence 
Signed-off-by: Josh Poimboeuf