Re: [PATCH 0/3] Unexport kallsyms_lookup_name() and kallsyms_on_each_symbol()

[Greg Kroah-Hartman]

On Mon, Mar 02, 2020 at 03:17:07PM -0500, Mathieu Desnoyers wrote:
> ----- On Mar 2, 2020, at 2:39 PM, Greg Kroah-Hartman gregkh@xxxxxxxxxxxxxxxxxxx wrote:
> 
> > On Mon, Mar 02, 2020 at 08:36:58PM +0100, Greg Kroah-Hartman wrote:
> >> On Mon, Mar 02, 2020 at 02:28:11PM -0500, Mathieu Desnoyers wrote:
> >> > On 21-Feb-2020 11:44:01 AM, Will Deacon wrote:
> >> > > Hi folks,
> >> > > 
> >> > > Despite having just a single modular in-tree user that I could spot,
> >> > > kallsyms_lookup_name() is exported to modules and provides a mechanism
> >> > > for out-of-tree modules to access and invoke arbitrary, non-exported
> >> > > kernel symbols when kallsyms is enabled.
> >> > > 
> >> > > This patch series fixes up that one user and unexports the symbol along
> >> > > with kallsyms_on_each_symbol(), since that could also be abused in a
> >> > > similar manner.
> >> > 
> >> > Hi,
> >> > 
> >> > I maintain a GPL kernel tracer (LTTng) since 2005 which happens to be
> >> > out-of-tree, even though we have made unsuccessful attempts to upstream
> >> > it in the past. It uses kallsyms_lookup_name() to fetch a few symbols. I
> >> > would be very glad to have them GPL-exported upstream rather than
> >> > relying on this work-around. Here is the list of symbols we would need
> >> > to GPL-export:
> >> > 
> >> > stack_trace_save
> >> > stack_trace_save_user
> >> > vmalloc_sync_all (CONFIG_X86)
> >> > get_pfnblock_flags_mask
> >> > disk_name
> >> > block_class
> >> > disk_type
> >> 
> >> I hate to ask, but why does anyone need block_class?  or disk_name or
> >> disk_type?  I need to put them behind a driver core namespace or
> >> something soon...
> > 
> 
> In LTTng, we have a "statedump" which dumps all the relevant state of
> the kernel at trace start (or when the user asks for it) into the
> kernel trace. It is used to collect information which helps translating
> compact numeric data into human-readable information at post-processing.
> 
> For block devices, the statedump contains the mapping between the
> device number and the disk name [1]. It uses the "block_class",
> "disk_name", and "disk_type" symbols for this. Here is the
> post-processing output:
> 
> [14:48:41.388934812] (+?.?????????) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 1048576, diskname = "ram0" }
> [...]
> [14:48:41.442548745] (+0.003574998) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 1048591, diskname = "ram15" }
> [14:48:41.446064977] (+0.003516232) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 265289728, diskname = "vda" }
> [14:48:41.449579781] (+0.003514804) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 265289729, diskname = "vda1" }
> [14:48:41.453113808] (+0.003534027) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 265289744, diskname = "vdb" }
> [14:48:41.456640876] (+0.003527068) compudjdev lttng_statedump_block_device: { cpu_id = 0 }, { dev = 265289745, diskname = "vdb1" }
> 
> This information is then used in our I/O analyses to show information
> comprehensible to a user.

But all of that is availble to you today in userspace, why dig through
random kernel symbols?

Look in /sys/dev/block/ or in /sys/block/ for all of that information.
Is there something that you can only find by the internal symbols that
is not present today in sysfs?

> > Wait, disk_type is a static variable.  And there's multiple ones of
> > them, how does that work?
> 
> Yes, this is far from ideal. Here are the ones I observe in the kernel
> sources:
> 
> block/genhd.c
> 40:static const struct device_type disk_type;   <---- the one we use
> 
> lib/raid6/test/test.c
> 41:static char disk_type(int d)  <---- this is a stand-alone user-space test program, not part of the kernel image.
> 
> crypto/async_tx/raid6test.c (depends on CONFIG_ASYNC_RAID6_TEST)
> 44:static char disk_type(int d, int disks) <---- the compiler optimizes away this function, so this symbol is not present in the kernel image.
> 
> I think a better approach to solve this would be to implement and expose an
> iterator function in the core kernel which could invoke a callback. However,
> the main issue remains: if the only user is out-of-tree, I cannot justify
> adding an exported kernel helper for this.

I think the best thing would be to use the userspace api :)

thanks,

greg k-h