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

    There is an additional complication with posix threads where the stack
    guard for a thread stack may be larger than a page, unlike the case
    for process stack where the stack guard is a page long. glibc
    implements these guards by calling mprotect on the beginning page(s)
    to remove all permissions. I have used this to remove vmas that have
    the thread stack guard, from the /proc/maps output.

    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 | 8 +++++---
    include/linux/mm.h | 17 +++++++++++++++++
    mm/mmap.c | 3 +++
    3 files changed, 25 insertions(+), 3 deletions(-)

    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index e418c5a..98b5275 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -227,7 +227,10 @@ 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 */
    + /* We don't show the stack guard pages in /proc/maps */
    + if (thread_stack_guard(vma))
    + return;
    +
    start = vma->vm_start;
    if (stack_guard_page_start(vma, start))
    start += PAGE_SIZE;
    @@ -259,8 +262,7 @@ 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)) {
    name = "[stack]";
    }
    } else {
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 17b27cd..9871e10 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -1018,6 +1018,23 @@ 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));
    +}
    +
    +/*
    + * POSIX thread stack guards may be more than a page long and access to it
    + * should return an error (possibly a SIGSEGV). The glibc implementation does
    + * an mprotect(..., ..., PROT_NONE), so our guard vma has no permissions.
    + */
    +static inline int thread_stack_guard(struct vm_area_struct *vma)
    +{
    + return vma_is_stack(vma) &&
    + ((vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC | VM_MAYSHARE)) == 0) &&
    + vma_is_stack((vma->vm_flags & VM_GROWSDOWN)?vma->vm_next:vma->vm_prev);
    +}
    +
    static inline int stack_guard_page_start(struct vm_area_struct *vma,
    unsigned long addr)
    {
    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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