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Subject[RFC] c/r: prctl: Add ability to set new mm_struct::exe_file
Hi guys,

at restore time we would like to have a way for /proc/pid/exe
symlink recovering. So I thought extending prctl might be
a good idea.

Still maybe there some other good and 'right' way to do it,
so I would like to gather opinions.

Please review, thanks!

Cyrill
---
From: Cyrill Gorcunov <gorcunov@openvz.org>
Subject: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file
When we do restore we would like to have a way to setup
a former mm_struct::exe_file so that /proc/pid/exe would
point to the original executable file a process had at
checkpoint time.

For this sake PR_SET_MM_EXE_FILE code is introduced.

Note, if mm_struct::exe_file already mapped more than once
we refuse to change anything (which prevents kernel from
potential problems).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
---
include/linux/prctl.h | 1
kernel/sys.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 73 insertions(+), 1 deletion(-)
Index: linux-2.6.git/include/linux/prctl.h
===================================================================
--- linux-2.6.git.orig/include/linux/prctl.h
+++ linux-2.6.git/include/linux/prctl.h
@@ -118,5 +118,6 @@
# define PR_SET_MM_ENV_START 10
# define PR_SET_MM_ENV_END 11
# define PR_SET_MM_AUXV 12
+# define PR_SET_MM_EXE_FILE 13

#endif /* _LINUX_PRCTL_H */
Index: linux-2.6.git/kernel/sys.c
===================================================================
--- linux-2.6.git.orig/kernel/sys.c
+++ linux-2.6.git/kernel/sys.c
@@ -1701,6 +1701,66 @@ static bool vma_flags_mismatch(struct vm
(vma->vm_flags & banned);
}

+/* Expects mm->mmap_sem is read-taken */
+static int prctl_set_mm_exe_file(struct mm_struct *mm,
+ const void __user *path,
+ size_t size)
+{
+ struct file *new_exe_file;
+ char *pathbuf;
+ int ret = 0;
+
+ if (size >= PATH_MAX)
+ return -EINVAL;
+
+ /*
+ * We allow to change only those exe's which
+ * are not mapped several times. This one
+ * is early test while mmap_sem is taken.
+ */
+ if (mm->num_exe_file_vmas > 1)
+ return -EBUSY;
+
+ up_read(&mm->mmap_sem);
+
+ pathbuf = kmalloc(size, GFP_TEMPORARY);
+ if (!pathbuf) {
+ ret = -ENOMEM;
+ goto err_down;
+ }
+
+ if (copy_from_user(pathbuf, path, size)) {
+ kfree(pathbuf);
+ ret = -EFAULT;
+ goto err_down;
+ }
+ pathbuf[size-1] = '\0';
+
+ new_exe_file = open_exec(pathbuf);
+ kfree(pathbuf);
+
+ down_read(&mm->mmap_sem);
+
+ if (IS_ERR(new_exe_file))
+ return PTR_ERR(new_exe_file);
+
+ /*
+ * We allow to change only those exe's which
+ * are not mapped several times.
+ */
+ if (mm->num_exe_file_vmas < 2) {
+ set_mm_exe_file(mm, new_exe_file);
+ ret = 0;
+ } else
+ ret = -EBUSY;
+
+ return ret;
+
+err_down:
+ down_read(&mm->mmap_sem);
+ return ret;
+}
+
static int prctl_set_mm(int opt, unsigned long addr,
unsigned long arg4, unsigned long arg5)
{
@@ -1709,7 +1769,9 @@ static int prctl_set_mm(int opt, unsigne
struct vm_area_struct *vma;
int error = 0;

- if (arg5 || (arg4 && opt != PR_SET_MM_AUXV))
+ if (arg5 || (arg4 &&
+ opt != PR_SET_MM_AUXV &&
+ opt != PR_SET_MM_EXE_FILE))
return -EINVAL;

if (!capable(CAP_SYS_ADMIN))
@@ -1837,6 +1899,15 @@ static int prctl_set_mm(int opt, unsigne

return 0;
}
+
+ /*
+ * This to restore /proc/self/exe link.
+ */
+ case PR_SET_MM_EXE_FILE:
+ error = prctl_set_mm_exe_file(mm, (const void __user *)addr, arg4);
+ if (error)
+ goto out;
+ break;
default:
error = -EINVAL;
goto out;

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