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    Subject[PATCH 2/3] proc: Stop trying to report thread stacks
    Date
    This reverts more of b76437579d13 ("procfs: mark thread stack
    correctly in proc/<pid>/maps"), which was partially reverted by
    65376df58217 ("proc: revert /proc/<pid>/maps [stack:TID]
    annotation").

    Originally, /proc/PID/task/TID/maps was the same as /proc/TID/maps.

    In current kernels, /proc/PID/maps (or /proc/TID/maps even for
    threads) shows "[stack]" for VMAs in the mm's stack address range.
    In contract, /proc/PID/task/TID/maps uses KSTK_ESP to guess the
    target thread's stack's VMA. This is racy, probably returns garbage
    and, on arches with CONFIG_TASK_INFO_IN_THREAD, crash-prone --
    KSTK_ESP is not safe to use on tasks that aren't known to be running
    ordinary process-context kernel code.

    This patch removes the difference and just shows "[stack]" for VMAs
    in the mm's stack range. This is IMO much more sensible -- the
    actual "stack" address really is treated specially by the VM code,
    and the current thread stack isn't even well-defined for programs
    that frequently switch stacks on their own.

    Reported-by: Jann Horn <jann@thejh.net>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    ---
    Documentation/filesystems/proc.txt | 26 --------------------------
    fs/proc/task_mmu.c | 29 ++++++++++-------------------
    fs/proc/task_nommu.c | 28 ++++++++++------------------
    3 files changed, 20 insertions(+), 63 deletions(-)

    diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
    index 68080ad6a75e..0457bf57d2eb 100644
    --- a/Documentation/filesystems/proc.txt
    +++ b/Documentation/filesystems/proc.txt
    @@ -395,32 +395,6 @@ is not associated with a file:

    or if empty, the mapping is anonymous.

    -The /proc/PID/task/TID/maps is a view of the virtual memory from the viewpoint
    -of the individual tasks of a process. In this file you will see a mapping marked
    -as [stack] if that task sees it as a stack. Hence, for the example above, the
    -task-level map, i.e. /proc/PID/task/TID/maps for thread 1001 will look like this:
    -
    -08048000-08049000 r-xp 00000000 03:00 8312 /opt/test
    -08049000-0804a000 rw-p 00001000 03:00 8312 /opt/test
    -0804a000-0806b000 rw-p 00000000 00:00 0 [heap]
    -a7cb1000-a7cb2000 ---p 00000000 00:00 0
    -a7cb2000-a7eb2000 rw-p 00000000 00:00 0
    -a7eb2000-a7eb3000 ---p 00000000 00:00 0
    -a7eb3000-a7ed5000 rw-p 00000000 00:00 0 [stack]
    -a7ed5000-a8008000 r-xp 00000000 03:00 4222 /lib/libc.so.6
    -a8008000-a800a000 r--p 00133000 03:00 4222 /lib/libc.so.6
    -a800a000-a800b000 rw-p 00135000 03:00 4222 /lib/libc.so.6
    -a800b000-a800e000 rw-p 00000000 00:00 0
    -a800e000-a8022000 r-xp 00000000 03:00 14462 /lib/libpthread.so.0
    -a8022000-a8023000 r--p 00013000 03:00 14462 /lib/libpthread.so.0
    -a8023000-a8024000 rw-p 00014000 03:00 14462 /lib/libpthread.so.0
    -a8024000-a8027000 rw-p 00000000 00:00 0
    -a8027000-a8043000 r-xp 00000000 03:00 8317 /lib/ld-linux.so.2
    -a8043000-a8044000 r--p 0001b000 03:00 8317 /lib/ld-linux.so.2
    -a8044000-a8045000 rw-p 0001c000 03:00 8317 /lib/ld-linux.so.2
    -aff35000-aff4a000 rw-p 00000000 00:00 0
    -ffffe000-fffff000 r-xp 00000000 00:00 0 [vdso]
    -
    The /proc/PID/smaps is an extension based on maps, showing the memory
    consumption for each of the process's mappings. For each of mappings there
    is a series of lines such as the following:
    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index f6fa99eca515..4e3a9510c9cc 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -264,24 +264,15 @@ static int do_maps_open(struct inode *inode, struct file *file,
    * /proc/PID/maps that is the stack of the main task.
    */
    static int is_stack(struct proc_maps_private *priv,
    - struct vm_area_struct *vma, int is_pid)
    + struct vm_area_struct *vma)
    {
    - int stack = 0;
    -
    - if (is_pid) {
    - stack = vma->vm_start <= vma->vm_mm->start_stack &&
    - vma->vm_end >= vma->vm_mm->start_stack;
    - } else {
    - struct inode *inode = priv->inode;
    - struct task_struct *task;
    -
    - rcu_read_lock();
    - task = pid_task(proc_pid(inode), PIDTYPE_PID);
    - if (task)
    - stack = vma_is_stack_for_task(vma, task);
    - rcu_read_unlock();
    - }
    - return stack;
    + /*
    + * We make no effort to guess what a given thread considers to be
    + * its "stack". It's not even well-defined for programs written
    + * languages like Go.
    + */
    + return vma->vm_start <= vma->vm_mm->start_stack &&
    + vma->vm_end >= vma->vm_mm->start_stack;
    }

    static void
    @@ -352,7 +343,7 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma, int is_pid)
    goto done;
    }

    - if (is_stack(priv, vma, is_pid))
    + if (is_stack(priv, vma))
    name = "[stack]";
    }

    @@ -1667,7 +1658,7 @@ static int show_numa_map(struct seq_file *m, void *v, int is_pid)
    seq_file_path(m, file, "\n\t= ");
    } else if (vma->vm_start <= mm->brk && vma->vm_end >= mm->start_brk) {
    seq_puts(m, " heap");
    - } else if (is_stack(proc_priv, vma, is_pid)) {
    + } else if (is_stack(proc_priv, vma)) {
    seq_puts(m, " stack");
    }

    diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
    index faacb0c0d857..37175621e890 100644
    --- a/fs/proc/task_nommu.c
    +++ b/fs/proc/task_nommu.c
    @@ -124,25 +124,17 @@ unsigned long task_statm(struct mm_struct *mm,
    }

    static int is_stack(struct proc_maps_private *priv,
    - struct vm_area_struct *vma, int is_pid)
    + struct vm_area_struct *vma)
    {
    struct mm_struct *mm = vma->vm_mm;
    - int stack = 0;
    -
    - if (is_pid) {
    - stack = vma->vm_start <= mm->start_stack &&
    - vma->vm_end >= mm->start_stack;
    - } else {
    - struct inode *inode = priv->inode;
    - struct task_struct *task;
    -
    - rcu_read_lock();
    - task = pid_task(proc_pid(inode), PIDTYPE_PID);
    - if (task)
    - stack = vma_is_stack_for_task(vma, task);
    - rcu_read_unlock();
    - }
    - return stack;
    +
    + /*
    + * We make no effort to guess what a given thread considers to be
    + * its "stack". It's not even well-defined for programs written
    + * languages like Go.
    + */
    + return vma->vm_start <= mm->start_stack &&
    + vma->vm_end >= mm->start_stack;
    }

    /*
    @@ -184,7 +176,7 @@ static int nommu_vma_show(struct seq_file *m, struct vm_area_struct *vma,
    if (file) {
    seq_pad(m, ' ');
    seq_file_path(m, file, "");
    - } else if (mm && is_stack(priv, vma, is_pid)) {
    + } else if (mm && is_stack(priv, vma)) {
    seq_pad(m, ' ');
    seq_printf(m, "[stack]");
    }
    --
    2.7.4
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