linux-kernel - [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

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Message-ID: <CAP045AoL4=6L16Nxi4TW7Jt8dqNXBQTvZZOqBEBRRcZdHCQ_VA@mail.gmail.com>
Date:   Sat, 30 Jan 2021 17:32:21 -0800
From:   Kyle Huey <me@...ehuey.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        "Robert O'Callahan" <rocallahan@...il.com>
Subject: [REGRESSION] x86/entry: TIF_SINGLESTEP handling is still broken

Yuxuan Shui previous reported a regression in single step reporting,
introduced in 64eb35f701f04b30706e21d1b02636b5d31a37d2, with a patch
to fix it.

However, after that is fixed, there is another regression introduced
later in the same series, in 2991552447707d791d9d81a5dc161f9e9e90b163,
that again breaks the same code.

The patch renames ARCH_SYSCALL_EXIT_WORK to ARCH_SYSCALL_WORK_EXIT,
which orphans the definition of ARCH_SYSCALL_EXIT_WORK in
arch/x86/include/asm/entry-common.h. No work was done to port
TIF_SINGLESTEP to syscall_work. Despite the code in report_single_step
that checks current_thread_info()->flags, because the code is no
longer checking the TIF values at all to decide whether to enter
syscall_exit_work, report_single_step will never be called and we will
again fail to report the single step.

I tested that with 2991552447707d791d9d81a5dc161f9e9e90b163 reverted
and Yuxuan's patch applied to Linus's tip rr works and passes all
tests.

- Kyle