file.lastModified() is never what was set with file.setLastModified()

Question

I do have a problem with millis set and read on Android 2.3.4 on a Nexus One. This is the code:

File fileFolder = new File(Environment.getExternalStorageDire         


        

Answer 1

On Jelly Bean+, it's different (mostly on Nexus devices yet, and others that use the new fuse layer for /mnt/shell/emulated sdcard emulation):

It's a VFS permission problem, the syscall utimensat() fails with EPERM due to inappropriate permissions (e.g. ownership).

in platform/system/core/sdcard/sdcard.c:

/* all files owned by root.sdcard */
attr->uid = 0;
attr->gid = AID_SDCARD_RW;

From utimensat()'s syscall man page:

2. the caller's effective user ID must match the owner of the file; or  
3. the caller must have appropriate privileges.  

To make any change other than setting both timestamps to the current time  
(i.e., times is not NULL, and both tv_nsec fields are not UTIME_NOW and both  
tv_nsec fields are not UTIME_OMIT), either condition 2 or 3 above must apply.  

Old FAT offers an override of the iattr->valid flag via a mount option to allow changing timestamps to anyone, FUSE+Android's sdcard-FUSE don't do this at the moment (so the 'inode_change_ok() call fails) and the attempt gets rejected with -EPERM. Here's FAT's ./fs/fat/file.c:

/* Check for setting the inode time. */  
ia_valid = attr->ia_valid;  
if (ia_valid & TIMES_SET_FLAGS) {  
    if (fat_allow_set_time(sbi, inode))  
        attr->ia_valid &= ~TIMES_SET_FLAGS;  
}  

error = inode_change_ok(inode, attr);

I also added this info to this open bug.