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SubjectRe: Deadlock on the mm->mmap_sem
Attached is a rewritten proc_pid_read_maps. Patch against 2.4.9-ac9, but
should apply against 2.4.9 as well.

As a side-effect, it's more efficient since it tries to return multiple
lines in each call, without the volatile_task hack.

Code that's older than 2.4.3 can't cause recursions - that was before
the rw-mmap_sem change.

* ptrace doesn't cause any recursions, it always copies into temporary
kernel buffers.

* the multithreaded coredump patch added down_read() around
binfmt->core_dump(). I'll think about the simplest way to fix that.

--
Manfred--- 2.4/fs/proc/array.c Thu Sep 6 20:51:37 2001
+++ build-2.4/fs/proc/array.c Tue Sep 18 14:25:17 2001
@@ -537,136 +537,142 @@
#define MAPS_LINE_FORMAT8 "%016lx-%016lx %s %016lx %s %lu"
#define MAPS_LINE_MAX8 73 /* sum of 16 1 16 1 4 1 16 1 5 1 10 1 */

-#define MAPS_LINE_MAX MAPS_LINE_MAX8
+#define MAPS_LINE_FORMAT (sizeof(void*) == 4 ? MAPS_LINE_FORMAT4 : MAPS_LINE_FORMAT8)
+#define MAPS_LINE_MAX (sizeof(void*) == 4 ? MAPS_LINE_MAX4 : MAPS_LINE_MAX8)

+int proc_pid_maps_get_line (char *buf, struct vm_area_struct *map)
+{
+ /* produce the next line */
+ char *line;
+ char str[5];
+ int flags;
+ kdev_t dev;
+ unsigned long ino;
+ int len;
+
+ flags = map->vm_flags;
+
+ str[0] = flags & VM_READ ? 'r' : '-';
+ str[1] = flags & VM_WRITE ? 'w' : '-';
+ str[2] = flags & VM_EXEC ? 'x' : '-';
+ str[3] = flags & VM_MAYSHARE ? 's' : 'p';
+ str[4] = 0;
+
+ dev = 0;
+ ino = 0;
+ if (map->vm_file != NULL) {
+ dev = map->vm_file->f_dentry->d_inode->i_dev;
+ ino = map->vm_file->f_dentry->d_inode->i_ino;
+ line = d_path(map->vm_file->f_dentry,
+ map->vm_file->f_vfsmnt,
+ buf, PAGE_SIZE);
+ buf[PAGE_SIZE-1] = '\n';
+ line -= MAPS_LINE_MAX;
+ if(line < buf)
+ line = buf;
+ } else
+ line = buf;
+
+ len = sprintf(line,
+ MAPS_LINE_FORMAT,
+ map->vm_start, map->vm_end, str, map->vm_pgoff << PAGE_SHIFT,
+ kdevname(dev), ino);
+
+ if(map->vm_file) {
+ int i;
+ for(i = len; i < MAPS_LINE_MAX; i++)
+ line[i] = ' ';
+ len = buf + PAGE_SIZE - line;
+ memmove(buf, line, len);
+ } else
+ line[len++] = '\n';
+ return len;
+}

ssize_t proc_pid_read_maps (struct task_struct *task, struct file * file, char * buf,
size_t count, loff_t *ppos)
{
struct mm_struct *mm;
struct vm_area_struct * map, * next;
- char * destptr = buf, * buffer;
- loff_t lineno;
- ssize_t column, i;
- int volatile_task;
+ char *tmp, *kbuf;
long retval;
+ int off, lineno, loff;

+ /* reject calls with out of range parameters immediately */
+ retval = 0;
+ if (*ppos > LONG_MAX)
+ goto out;
+ if (count == 0)
+ goto out;
+ off = (long)*ppos;
/*
* We might sleep getting the page, so get it first.
*/
retval = -ENOMEM;
- buffer = (char*)__get_free_page(GFP_KERNEL);
- if (!buffer)
+ kbuf = (char*)__get_free_page(GFP_KERNEL);
+ if (!kbuf)
goto out;

- if (count == 0)
- goto getlen_out;
+ tmp = (char*)__get_free_page(GFP_KERNEL);
+ if (!tmp)
+ goto out_free1;
+
task_lock(task);
mm = task->mm;
if (mm)
atomic_inc(&mm->mm_users);
task_unlock(task);
+ retval = 0;
if (!mm)
- goto getlen_out;
-
- /* Check whether the mmaps could change if we sleep */
- volatile_task = (task != current || atomic_read(&mm->mm_users) > 2);
-
- /* decode f_pos */
- lineno = *ppos >> MAPS_LINE_SHIFT;
- column = *ppos & (MAPS_LINE_LENGTH-1);
+ goto out_free2;

- /* quickly go to line lineno */
down_read(&mm->mmap_sem);
- for (map = mm->mmap, i = 0; map && (i < lineno); map = map->vm_next, i++)
- continue;
-
- for ( ; map ; map = next ) {
- /* produce the next line */
- char *line;
- char str[5], *cp = str;
- int flags;
- kdev_t dev;
- unsigned long ino;
- int maxlen = (sizeof(void*) == 4) ?
- MAPS_LINE_MAX4 : MAPS_LINE_MAX8;
+ map = mm->mmap;
+ lineno = 0;
+ loff = 0;
+ if (count > PAGE_SIZE)
+ count = PAGE_SIZE;
+ while (map) {
int len;
-
- /*
- * Get the next vma now (but it won't be used if we sleep).
- */
- next = map->vm_next;
- flags = map->vm_flags;
-
- *cp++ = flags & VM_READ ? 'r' : '-';
- *cp++ = flags & VM_WRITE ? 'w' : '-';
- *cp++ = flags & VM_EXEC ? 'x' : '-';
- *cp++ = flags & VM_MAYSHARE ? 's' : 'p';
- *cp++ = 0;
-
- dev = 0;
- ino = 0;
- if (map->vm_file != NULL) {
- dev = map->vm_file->f_dentry->d_inode->i_dev;
- ino = map->vm_file->f_dentry->d_inode->i_ino;
- line = d_path(map->vm_file->f_dentry,
- map->vm_file->f_vfsmnt,
- buffer, PAGE_SIZE);
- buffer[PAGE_SIZE-1] = '\n';
- line -= maxlen;
- if(line < buffer)
- line = buffer;
- } else
- line = buffer;
-
- len = sprintf(line,
- sizeof(void*) == 4 ? MAPS_LINE_FORMAT4 : MAPS_LINE_FORMAT8,
- map->vm_start, map->vm_end, str, map->vm_pgoff << PAGE_SHIFT,
- kdevname(dev), ino);
-
- if(map->vm_file) {
- for(i = len; i < maxlen; i++)
- line[i] = ' ';
- len = buffer + PAGE_SIZE - line;
- } else
- line[len++] = '\n';
- if (column >= len) {
- column = 0; /* continue with next line at column 0 */
- lineno++;
- continue; /* we haven't slept */
+ if (off > MAPS_LINE_LENGTH) {
+ off -= MAPS_LINE_LENGTH;
+ goto next;
}
-
- i = len-column;
- if (i > count)
- i = count;
- copy_to_user(destptr, line+column, i); /* may have slept */
- destptr += i;
- count -= i;
- column += i;
- if (column >= len) {
- column = 0; /* next time: next line at column 0 */
- lineno++;
+ len = proc_pid_maps_get_line(tmp, map);
+ len -= off;
+ if (len > 0) {
+ if (retval+len > count) {
+ /* only partial line transfer possible */
+ len = count - retval;
+ /* save the offset where the next read
+ * must start */
+ loff = len+off;
+ }
+ memcpy(kbuf+retval, tmp+off, len);
+ retval += len;
}
-
- /* done? */
- if (count == 0)
- break;
-
- /* By writing to user space, we might have slept.
- * Stop the loop, to avoid a race condition.
- */
- if (volatile_task)
+ off = 0;
+next:
+ if (!loff)
+ lineno++;
+ if (retval >= count)
break;
+ if (loff) BUG();
+ map = map->vm_next;
+ }
+ if (retval > count) BUG();
+ if (copy_to_user(buf, kbuf, retval)) {
+ retval = -EFAULT;
+ } else {
+ *ppos = (lineno << MAPS_LINE_SHIFT) + loff;
}
up_read(&mm->mmap_sem);
-
- /* encode f_pos */
- *ppos = (lineno << MAPS_LINE_SHIFT) + column;
mmput(mm);

-getlen_out:
- retval = destptr - buf;
- free_page((unsigned long)buffer);
+out_free2:
+ free_page((unsigned long)tmp);
+out_free1:
+ free_page((unsigned long)kbuf);
out:
return retval;
}
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