uprobes: Teach copy_insn() to support tmpfs

tmpfs is widely used but as Denys reports shmem_aops doesn't have
->readpage() and thus you can't probe a binary on this filesystem.

As Hugh suggested we can use shmem_read_mapping_page() in this case,
just we need to check shmem_mapping() if ->readpage == NULL.

Reported-by: Denys Vlasenko <dvlasenk@redhat.com>
Suggested-by: Hugh Dickins <hughd@google.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index c56b13e..6bfb671 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -36,6 +36,7 @@
 #include "../../mm/internal.h" /* munlock_vma_page */
 #include <linux/percpu-rwsem.h>
 #include <linux/task_work.h>
+#include <linux/shmem_fs.h>
 
 #include <linux/uprobes.h>
 
@@ -537,10 +538,14 @@ static int __copy_insn(struct address_space *mapping, struct file *filp,
 {
        struct page *page;
        /*
-        * Ensure that the page that has the original instruction is
-        * populated and in page-cache.
+        * Ensure that the page that has the original instruction is populated
+        * and in page-cache. If ->readpage == NULL it must be shmem_mapping(),
+        * see uprobe_register().
         */
-       page = read_mapping_page(mapping, offset >> PAGE_CACHE_SHIFT, filp);
+       if (mapping->a_ops->readpage)
+               page = read_mapping_page(mapping, offset >> PAGE_CACHE_SHIFT, filp);
+       else
+               page = shmem_read_mapping_page(mapping, offset >> PAGE_CACHE_SHIFT);
        if (IS_ERR(page))
                return PTR_ERR(page);
 
@@ -876,8 +881,8 @@ int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *
        if (!uc->handler && !uc->ret_handler)
                return -EINVAL;
 
-       /* copy_insn()->read_mapping_page() needs ->readpage() */
-       if (!inode->i_mapping->a_ops->readpage)
+       /* copy_insn() uses read_mapping_page() or shmem_read_mapping_page() */
+       if (!inode->i_mapping->a_ops->readpage && !shmem_mapping(inode->i_mapping))
                return -EIO;
        /* Racy, just to catch the obvious mistakes */
        if (offset > i_size_read(inode))