On (12/04/16 12:28), Greg KH wrote: > Date: Sun, 4 Dec 2016 12:28:20 +0100 > From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> > To: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, Minchan Kim > <minchan@xxxxxxxxxx>, Steven Allen <steven@xxxxxxxxxxxxx>, > linux-kernel@xxxxxxxxxxxxxxx, stable@xxxxxxxxxxxxxxx, Sergey Senozhatsky > <sergey.senozhatsky.work@xxxxxxxxx> > Subject: Re: [PATCH] zram: restrict add/remove attributes to root only > User-Agent: Mutt/1.7.1 (2016-10-04) > > On Sun, Dec 04, 2016 at 07:52:08PM +0900, Sergey Senozhatsky wrote: > > On (12/04/16 11:28), Greg KH wrote: > > > On Sun, Dec 04, 2016 at 11:35:15AM +0900, Sergey Senozhatsky wrote: > > [..] > > > > > Why can't a normal user read the attribute? Does a read actually modify > > > something? > > > > yes, it does. to clarify a bit more: we allocate a new device ID using idr_alloc(). so the IDs are limited and, thus, the number of devices is limited as well - signed int. each new device has NO: -- zspoll (zsmalloc pool in zram case) -- compression per-CPU backends (working-mem/scratch buffers, etc.) -- meta table so no big memory allocations. (a 'normal' user can't init the device, he/she can just create it. which is the problem here: we don't want a 'normal' user be able to do this). every device has: -- blk queue -- sysfs attrs -- gendisk -- zram structure allocated. so each new device consumes some memory, but not insane amounts of it. > Oh that's totally and completely broken then. > > Reading from a sysfs file should NEVER cause side affects to the system. > Please fix up this api. some history. we started with a 'loop device'-like scheme, but ended up with a sysfs approach [1] https://marc.info/?l=linux-kernel&m=142495984002611 [2] https://marc.info/?l=linux-kernel&m=142507747808572 [3] https://marc.info/?l=linux-kernel&m=142530591720172 [4] https://marc.info/?l=linux-kernel&m=142509446812318 [5] https://marc.info/?l=linux-kernel&m=142509782112819 > > reading from a hot_add file creates a new zram device and returns a new > > device's device_id. not initialized device (so it does not eat the memory > > for handle table, etc.), but with its own set of sysfs attrs, etc. which > > consumes memory after all. so a 'normal' user, doing a simple read from a > > hot_add file in a loop just for fun, can create a lot of devices and, > > quite likely, cause some troubles (as reported by Steven Allen). > > Please switch this to be a char device node if you wish to "write and > get a device handle back". I don't know how I missed that in the > original api review, sorry about that. > > For now, you need to document the heck out of this in the attribute > declaration that this is what is going on. Otherwise someone like me > will come along and "fix up" the file to use ATTR_RO again in the > future and you will have the same problem again. I believe we have a documentation Documentation/ABI/testing/sysfs-class-zram and Documentation/blockdev/zram.txt both explain this attr. -ss -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html