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    Subject[PATCH 4.4 060/158] tracing/snapshot: Resize spare buffer if size changed
    Date
    From: Eiichi Tsukata <devel@etsukata.com>

    commit 46cc0b44428d0f0e81f11ea98217fc0edfbeab07 upstream.

    Current snapshot implementation swaps two ring_buffers even though their
    sizes are different from each other, that can cause an inconsistency
    between the contents of buffer_size_kb file and the current buffer size.

    For example:

    # cat buffer_size_kb
    7 (expanded: 1408)
    # echo 1 > events/enable
    # grep bytes per_cpu/cpu0/stats
    bytes: 1441020
    # echo 1 > snapshot // current:1408, spare:1408
    # echo 123 > buffer_size_kb // current:123, spare:1408
    # echo 1 > snapshot // current:1408, spare:123
    # grep bytes per_cpu/cpu0/stats
    bytes: 1443700
    # cat buffer_size_kb
    123 // != current:1408

    And also, a similar per-cpu case hits the following WARNING:

    Reproducer:

    # echo 1 > per_cpu/cpu0/snapshot
    # echo 123 > buffer_size_kb
    # echo 1 > per_cpu/cpu0/snapshot

    WARNING:

    WARNING: CPU: 0 PID: 1946 at kernel/trace/trace.c:1607 update_max_tr_single.part.0+0x2b8/0x380
    Modules linked in:
    CPU: 0 PID: 1946 Comm: bash Not tainted 5.2.0-rc6 #20
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-2.fc30 04/01/2014
    RIP: 0010:update_max_tr_single.part.0+0x2b8/0x380
    Code: ff e8 dc da f9 ff 0f 0b e9 88 fe ff ff e8 d0 da f9 ff 44 89 ee bf f5 ff ff ff e8 33 dc f9 ff 41 83 fd f5 74 96 e8 b8 da f9 ff <0f> 0b eb 8d e8 af da f9 ff 0f 0b e9 bf fd ff ff e8 a3 da f9 ff 48
    RSP: 0018:ffff888063e4fca0 EFLAGS: 00010093
    RAX: ffff888066214380 RBX: ffffffff99850fe0 RCX: ffffffff964298a8
    RDX: 0000000000000000 RSI: 00000000fffffff5 RDI: 0000000000000005
    RBP: 1ffff1100c7c9f96 R08: ffff888066214380 R09: ffffed100c7c9f9b
    R10: ffffed100c7c9f9a R11: 0000000000000003 R12: 0000000000000000
    R13: 00000000ffffffea R14: ffff888066214380 R15: ffffffff99851060
    FS: 00007f9f8173c700(0000) GS:ffff88806d000000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 0000000000714dc0 CR3: 0000000066fa6000 CR4: 00000000000006f0
    Call Trace:
    ? trace_array_printk_buf+0x140/0x140
    ? __mutex_lock_slowpath+0x10/0x10
    tracing_snapshot_write+0x4c8/0x7f0
    ? trace_printk_init_buffers+0x60/0x60
    ? selinux_file_permission+0x3b/0x540
    ? tracer_preempt_off+0x38/0x506
    ? trace_printk_init_buffers+0x60/0x60
    __vfs_write+0x81/0x100
    vfs_write+0x1e1/0x560
    ksys_write+0x126/0x250
    ? __ia32_sys_read+0xb0/0xb0
    ? do_syscall_64+0x1f/0x390
    do_syscall_64+0xc1/0x390
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

    This patch adds resize_buffer_duplicate_size() to check if there is a
    difference between current/spare buffer sizes and resize a spare buffer
    if necessary.

    Link: http://lkml.kernel.org/r/20190625012910.13109-1-devel@etsukata.com

    Cc: stable@vger.kernel.org
    Fixes: ad909e21bbe69 ("tracing: Add internal tracing_snapshot() functions")
    Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Nobuhiro Iwamatsu <nobuhiro1.iwamatsu@toshiba.co.jp>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/trace/trace.c | 12 ++++++++----
    1 file changed, 8 insertions(+), 4 deletions(-)

    --- a/kernel/trace/trace.c
    +++ b/kernel/trace/trace.c
    @@ -5406,11 +5406,15 @@ tracing_snapshot_write(struct file *filp
    break;
    }
    #endif
    - if (!tr->allocated_snapshot) {
    + if (!tr->allocated_snapshot)
    + ret = resize_buffer_duplicate_size(&tr->max_buffer,
    + &tr->trace_buffer, iter->cpu_file);
    + else
    ret = alloc_snapshot(tr);
    - if (ret < 0)
    - break;
    - }
    +
    + if (ret < 0)
    + break;
    +
    local_irq_disable();
    /* Now, we're going to swap */
    if (iter->cpu_file == RING_BUFFER_ALL_CPUS)

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