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    Subject[PATCH] Mark thread stack correctly in proc/<pid>/maps
    Date
    [Take 2]

    Memory mmaped by glibc for a thread stack currently shows up as a simple
    anonymous map, which makes it difficult to differentiate between memory
    usage of the thread on stack and other dynamic allocation. Since glibc
    already uses MAP_STACK to request this mapping, the attached patch
    uses this flag to add additional VM_STACK_FLAGS to the resulting vma
    so that the mapping is treated as a stack and not any regular
    anonymous mapping. Also, one may use vm_flags to decide if a vma is a
    stack.

    This patch also changes the maps output to annotate stack guards for
    both the process stack as well as the thread stacks. Thus is born the
    [stack guard] annotation, which should be exactly a page long for the
    process stack and can be longer than a page (configurable in
    userspace) for POSIX compliant thread stacks. A thread stack guard is
    simply page(s) with PROT_NONE.

    If accepted, this should also reflect in the man page for mmap since
    MAP_STACK will no longer be a noop.

    Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
    ---
    fs/proc/task_mmu.c | 41 ++++++++++++++++++++++++++++++++++++-----
    include/linux/mm.h | 19 +++++++++++++++++--
    mm/mmap.c | 3 +++
    3 files changed, 56 insertions(+), 7 deletions(-)

    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index e418c5a..650330c 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -227,13 +227,42 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
    pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
    }

    - /* We don't show the stack guard page in /proc/maps */
    + /*
    + * Mark the process stack guard, which is just one page at the
    + * beginning of the stack within the stack vma.
    + */
    start = vma->vm_start;
    - if (stack_guard_page_start(vma, start))
    + if (stack_guard_page_start(vma, start)) {
    + seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
    + start,
    + start + PAGE_SIZE,
    + flags & VM_READ ? 'r' : '-',
    + flags & VM_WRITE ? 'w' : '-',
    + flags & VM_EXEC ? 'x' : '-',
    + flags & VM_MAYSHARE ? 's' : 'p',
    + pgoff,
    + MAJOR(dev), MINOR(dev), ino, &len);
    +
    + pad_len_spaces(m, len);
    + seq_puts(m, "[stack guard]\n");
    start += PAGE_SIZE;
    + }
    end = vma->vm_end;
    - if (stack_guard_page_end(vma, end))
    + if (stack_guard_page_end(vma, end)) {
    + seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
    + end - PAGE_SIZE,
    + end,
    + flags & VM_READ ? 'r' : '-',
    + flags & VM_WRITE ? 'w' : '-',
    + flags & VM_EXEC ? 'x' : '-',
    + flags & VM_MAYSHARE ? 's' : 'p',
    + pgoff,
    + MAJOR(dev), MINOR(dev), ino, &len);
    +
    + pad_len_spaces(m, len);
    + seq_puts(m, "[stack guard]\n");
    end -= PAGE_SIZE;
    + }

    seq_printf(m, "%08lx-%08lx %c%c%c%c %08llx %02x:%02x %lu %n",
    start,
    @@ -259,8 +288,10 @@ static void show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
    if (vma->vm_start <= mm->brk &&
    vma->vm_end >= mm->start_brk) {
    name = "[heap]";
    - } else if (vma->vm_start <= mm->start_stack &&
    - vma->vm_end >= mm->start_stack) {
    + } else if (vma_is_stack(vma) &&
    + vma_is_guard(vma)) {
    + name = "[stack guard]";
    + } else if (vma_is_stack(vma)) {
    name = "[stack]";
    }
    } else {
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 17b27cd..4e57753 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -1018,12 +1018,26 @@ static inline int vma_growsdown(struct vm_area_struct *vma, unsigned long addr)
    return vma && (vma->vm_end == addr) && (vma->vm_flags & VM_GROWSDOWN);
    }

    +static inline int vma_is_stack(struct vm_area_struct *vma)
    +{
    + return vma && (vma->vm_flags & (VM_GROWSUP | VM_GROWSDOWN));
    +}
    +
    +/*
    + * Check guard set by userspace (PROT_NONE)
    + */
    +static inline int vma_is_guard(struct vm_area_struct *vma)
    +{
    + return (vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC | VM_SHARED)) == 0;
    +}
    +
    static inline int stack_guard_page_start(struct vm_area_struct *vma,
    unsigned long addr)
    {
    return (vma->vm_flags & VM_GROWSDOWN) &&
    (vma->vm_start == addr) &&
    - !vma_growsdown(vma->vm_prev, addr);
    + !vma_growsdown(vma->vm_prev, addr) &&
    + !vma_is_guard(vma);
    }

    /* Is the vma a continuation of the stack vma below it? */
    @@ -1037,7 +1051,8 @@ static inline int stack_guard_page_end(struct vm_area_struct *vma,
    {
    return (vma->vm_flags & VM_GROWSUP) &&
    (vma->vm_end == addr) &&
    - !vma_growsup(vma->vm_next, addr);
    + !vma_growsup(vma->vm_next, addr) &&
    + !vma_is_guard(vma);
    }

    extern unsigned long move_page_tables(struct vm_area_struct *vma,
    diff --git a/mm/mmap.c b/mm/mmap.c
    index 3f758c7..2f9f540 100644
    --- a/mm/mmap.c
    +++ b/mm/mmap.c
    @@ -992,6 +992,9 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
    vm_flags = calc_vm_prot_bits(prot) | calc_vm_flag_bits(flags) |
    mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC;

    + if (flags & MAP_STACK)
    + vm_flags |= VM_STACK_FLAGS;
    +
    if (flags & MAP_LOCKED)
    if (!can_do_mlock())
    return -EPERM;
    --
    1.7.7.4


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