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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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