[PATCH 2/2] mmap: allow MAP_HUGETLB for hugetlbfs files

[Joern Engel <joern@logfs.org>]

It is counterintuitive at best that mmap'ing a hugetlbfs file with
MAP_HUGETLB fails, while mmap'ing it without will a) succeed and b)
return huge pages.

Signed-off-by: Joern Engel <joern@logfs.org>
---
 mm/mmap.c |   12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 2a594246..76eb6df 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -33,6 +33,7 @@
 #include <linux/uprobes.h>
 #include <linux/rbtree_augmented.h>
 #include <linux/sched/sysctl.h>
+#include <linux/magic.h>
 
 #include <asm/uaccess.h>
 #include <asm/cacheflush.h>
@@ -1313,6 +1314,11 @@ unsigned long do_mmap_pgoff(struct file *file, unsigned long addr,
 	return addr;
 }
 
+static inline int is_hugetlb_file(struct file *file)
+{
+	return file->f_inode->i_sb->s_magic == HUGETLBFS_MAGIC;
+}
+
 SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 		unsigned long, prot, unsigned long, flags,
 		unsigned long, fd, unsigned long, pgoff)
@@ -1322,11 +1328,12 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 
 	if (!(flags & MAP_ANONYMOUS)) {
 		audit_mmap_fd(fd, flags);
-		if (unlikely(flags & MAP_HUGETLB))
-			return -EINVAL;
 		file = fget(fd);
 		if (!file)
 			goto out;
+		retval = -EINVAL;
+		if (unlikely(flags & MAP_HUGETLB && !is_hugetlb_file(file)))
+			goto out_fput;
 	} else if (flags & MAP_HUGETLB) {
 		struct user_struct *user = NULL;
 		/*
@@ -1346,6 +1353,7 @@ SYSCALL_DEFINE6(mmap_pgoff, unsigned long, addr, unsigned long, len,
 	flags &= ~(MAP_EXECUTABLE | MAP_DENYWRITE);
 
 	retval = vm_mmap_pgoff(file, addr, len, prot, flags, pgoff);
+out_fput:
 	if (file)
 		fput(file);
 out:
-- 
1.7.10.4

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