Re: [PATCH] tracing: Have type enum modifications copy the strings

[Marc Zyngier]

On Fri, 18 Mar 2022 19:34:32 +0000,
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> 
> From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>
> 
> When an enum is used in the visible parts of a trace event that is
> exported to user space, the user space applications like perf and
> trace-cmd do not have a way to know what the value of the enum is. To
> solve this, at boot up (or module load) the printk formats are modified to
> replace the enum with their numeric value in the string output.
> 
> Array fields of the event are defined by [<nr-elements>] in the type
> portion of the format file so that the user space parsers can correctly
> parse the array into the appropriate size chunks. But in some trace
> events, an enum is used in defining the size of the array, which once
> again breaks the parsing of user space tooling.
> 
> This was solved the same way as the print formats were, but it modified
> the type strings of the trace event. This caused crashes in some
> architectures because, as supposed to the print string, is a const string
> value. This was not detected on x86, as it appears that const strings are
> still writable (at least in boot up), but other architectures this is not
> the case, and writing to a const string will cause a kernel fault.
> 
> To fix this, use kstrdup() to copy the type before modifying it. If the
> trace event is for the core kernel there's no need to free it because the
> string will be in use for the life of the machine being on line. For
> modules, create a link list to store all the strings being allocated for
> modules and when the module is removed, free them.
> 
> Link: https://lore.kernel.org/all/yt9dr1706b4i.fsf@xxxxxxxxxxxxx/
> 
> Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well")
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

This fixes booting on arm64 with ext4 as a module, so FWIW:

Tested-by: Marc Zyngier <maz@xxxxxxxxxx>

	M.

-- 
Without deviation from the norm, progress is not possible.