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

From: Sven Schnelle
Date: Sun Mar 20 2022 - 09:15:29 EST

Hi Steve,

Steven Rostedt <rostedt@xxxxxxxxxxx> writes:

> 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:
> Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well")
> Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

This fixes the crash seen on s390. Thanks!

Tested-by: Sven Schnelle <svens@xxxxxxxxxxxxx>