linux-kernel - [PATCH 0/3] static_call/x86: Handle clang's conditional tail calls

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Date:   Mon, 23 Jan 2023 21:59:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org
Cc:     tglx@...utronix.de, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, peterz@...radead.org,
        jpoimboe@...nel.org, jbaron@...mai.com, rostedt@...dmis.org,
        ardb@...nel.org, linux-kernel@...r.kernel.org,
        erhard_f@...lbox.org, ndesaulniers@...gle.com, mhiramat@...nel.org,
        sandipan.das@....com
Subject: [PATCH 0/3] static_call/x86: Handle clang's conditional tail calls

Erhard reported boot fails on this AMD machine when using clang and bisected it
to a commit introducing a few static_call()s. Turns out that when using clang
with -Os it it very likely to generate conditional tail calls like:

  0000000000000350 <amd_pmu_add_event>:
  350:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1) 351: R_X86_64_NONE      __fentry__-0x4
  355:       48 83 bf 20 01 00 00 00         cmpq   $0x0,0x120(%rdi)
  35d:       0f 85 00 00 00 00       jne    363 <amd_pmu_add_event+0x13>     35f: R_X86_64_PLT32     __SCT__amd_pmu_branch_add-0x4
  363:       e9 00 00 00 00          jmp    368 <amd_pmu_add_event+0x18>     364: R_X86_64_PLT32     __x86_return_thunk-0x4

And our inline static_call() patching code can't deal with those and BUG
happens -- really early.

These patches borrow the kprobe Jcc emulation to implement text_poke_bp() Jcc
support, which is then used to teach inline static_call() about this form.

---
 arch/x86/include/asm/text-patching.h | 31 ++++++++++++++++++
 arch/x86/kernel/alternative.c        | 62 +++++++++++++++++++++++++++---------
 arch/x86/kernel/kprobes/core.c       | 38 +++++-----------------
 arch/x86/kernel/static_call.c        | 50 +++++++++++++++++++++++++++--
 4 files changed, 133 insertions(+), 48 deletions(-)