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

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Message-ID: <yt9d5yo8rcjd.fsf@linux.ibm.com>
Date:   Sun, 20 Mar 2022 14:14:46 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        Ritesh Harjani <riteshh@...ux.ibm.com>,
        Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>
Subject: Re: [PATCH] tracing: Have type enum modifications copy the strings

Hi Steve,

Steven Rostedt <rostedt@...dmis.org> writes:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
>
> 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@linux.ibm.com/
>
> Fixes: b3bc8547d3be ("tracing: Have TRACE_DEFINE_ENUM affect trace event types as well")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

This fixes the crash seen on s390. Thanks!

Tested-by: Sven Schnelle <svens@...ux.ibm.com>