perf/core: Fix MMAP2 event device with backing files

Some file systems like FUSE-based ones or overlayfs may record the backing
file in struct vm_area_struct vm_file, instead of the user file that the
user mmapped.

That causes perf to misreport the device major/minor numbers of the file
system of the file, and the generation of the file, and potentially other
inode details.  There is an existing helper file_user_inode() for that
situation.

Use file_user_inode() instead of file_inode() to get the inode for MMAP2
events.

Example:

  Setup:

    # cd /root
    # mkdir test ; cd test ; mkdir lower upper work merged
    # cp `which cat` lower
    # mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merged
    # perf record -e cycles:u -- /root/test/merged/cat /proc/self/maps
    ...
    55b2c91d0000-55b2c926b000 r-xp 00018000 00:1a 3419                       /root/test/merged/cat
    ...
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.004 MB perf.data (5 samples) ]
    #
    # stat /root/test/merged/cat
      File: /root/test/merged/cat
      Size: 1127792         Blocks: 2208       IO Block: 4096   regular file
    Device: 0,26    Inode: 3419        Links: 1
    Access: (0755/-rwxr-xr-x)  Uid: (    0/    root)   Gid: (    0/    root)
    Access: 2025-09-08 12:23:59.453309624 +0000
    Modify: 2025-09-08 12:23:59.454309624 +0000
    Change: 2025-09-08 12:23:59.454309624 +0000
     Birth: 2025-09-08 12:23:59.453309624 +0000

  Before:

    Device reported 00:02 differs from stat output and /proc/self/maps

    # perf script --show-mmap-events | grep /root/test/merged/cat
             cat     377 [-01]   243.078558: PERF_RECORD_MMAP2 377/377: [0x55b2c91d0000(0x9b000) @ 0x18000 00:02 3419 2068525940]: r-xp /root/test/merged/cat

  After:

    Device reported 00:1a is the same as stat output and /proc/self/maps

    # perf script --show-mmap-events | grep /root/test/merged/cat
             cat     362 [-01]   127.755167: PERF_RECORD_MMAP2 362/362: [0x55ba6e781000(0x9b000) @ 0x18000 00:1a 3419 0]: r-xp /root/test/merged/cat

With respect to stable kernels, overlayfs mmap function ovl_mmap() was
added in v4.19 but file_user_inode() was not added until v6.8 and never
back-ported to stable kernels.  FMODE_BACKING that it depends on was added
in v6.5.  This issue has gone largely unnoticed, so back-porting before
v6.8 is probably not worth it, so put 6.8 as the stable kernel prerequisite
version, although in practice the next long term kernel is 6.12.

Signed-off-by: Adrian Hunter <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Acked-by: Amir Goldstein <[email protected]>
Cc: [email protected] # 6.8

Author: ahunter6

kernel/events/core.c

@@ -9403,7 +9403,7 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
 		flags |= MAP_HUGETLB;
 
 	if (file) {
-		struct inode *inode;
+		const struct inode *inode;
 		dev_t dev;
 
 		buf = kmalloc(PATH_MAX, GFP_KERNEL);
@@ -9421,7 +9421,7 @@ static void perf_event_mmap_event(struct perf_mmap_event *mmap_event)
 			name = "//toolong";
 			goto cpy_name;
 		}
-		inode = file_inode(vma->vm_file);
+		inode = file_user_inode(vma->vm_file);
 		dev = inode->i_sb->s_dev;
 		ino = inode->i_ino;
 		gen = inode->i_generation;