Fix: module signature vs tracepoints: add new TAINT_UNSIGNED_MODULE

Users have reported being unable to trace non-signed modules loaded
within a kernel supporting module signature.

This is caused by tracepoint.c:tracepoint_module_coming() refusing to
take into account tracepoints sitting within force-loaded modules
(TAINT_FORCED_MODULE). The reason for this check, in the first place, is
that a force-loaded module may have a struct module incompatible with
the layout expected by the kernel, and can thus cause a kernel crash
upon forced load of that module on a kernel with CONFIG_TRACEPOINTS=y.

Tracepoints, however, specifically accept TAINT_OOT_MODULE and
TAINT_CRAP, since those modules do not lead to the "very likely system
crash" issue cited above for force-loaded modules.

With kernels having CONFIG_MODULE_SIG=y (signed modules), a non-signed
module is tainted re-using the TAINT_FORCED_MODULE taint flag.
Unfortunately, this means that Tracepoints treat that module as a
force-loaded module, and thus silently refuse to consider any tracepoint
within this module.

Since an unsigned module does not fit within the "very likely system
crash" category of tainting, add a new TAINT_UNSIGNED_MODULE taint flag
to specifically address this taint behavior, and accept those modules
within Tracepoints. We use the letter 'X' as a taint flag character for
a module being loaded that doesn't know how to sign its name (proposed
by Steven Rostedt).

Also add the missing 'O' entry to trace event show_module_flags() list
for the sake of completeness.

Signed-off-by: Mathieu Desnoyers <[email protected]>
Acked-by: Steven Rostedt <[email protected]>
NAKed-by: Ingo Molnar <[email protected]>
CC: Thomas Gleixner <[email protected]>
CC: David Howells <[email protected]>
CC: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Rusty Russell <[email protected]>

Documentation/sysctl/kernel.txt

@@ -792,6 +792,8 @@ can be ORed together:
 1024 - A module from drivers/staging was loaded.
 2048 - The system is working around a severe firmware bug.
 4096 - An out-of-tree module has been loaded.
+8192 - An unsigned module has been loaded in a kernel supporting module
+       signature.
 
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