Re: [PATCH] selftests/ftrace: Differentiate bash and dash in dynevent_limitations.tc

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 14 Apr 2025 21:09:00 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Steven Rostedt <rostedt@goodmis.org>
> 
> bash and dash evaluate variables differently.
> dash will evaluate '\\' every time it is read whereas bash does not.
> 
>   TEST_STRING="$TEST_STRING \\$i"
>   echo $TEST_STRING
> 
> With i=123
> On bash, that will print "\123"
> but on dash, that will print the escape sequence of \123 as the \ will be
> interpreted again in the echo.
> 
> The dynevent_limitations.tc test created a very large list of arguments to
> test the maximum number of arguments to pass to the dynamic events file.
> It had a loop of:
> 
>    TEST_STRING=$1
>    # Acceptable
>    for i in `seq 1 $MAX_ARGS`; do
>      TEST_STRING="$TEST_STRING \\$i"
>    done
>    echo "$TEST_STRING" >> dynamic_events
> 
> This worked fine on bash, but when run on dash it failed.
> 
> This was due to dash interpreting the "\\$i" twice. Once when it was
> assigned to TEST_STRING and a second time with the echo $TEST_STRING.
> 
> bash does not process the backslash more than the first time.
> 
> To solve this, assign a double backslash to a variable "bs" and then echo
> it to "ts". If "ts" changes, it is dash, if not, it is bash. Then update
> "bs" accordingly, and use that to assign TEST_STRING.
> 
> Now this could possibly just check if "$BASH" is defined or not, but this
> is testing if the issue exists and not just which shell is being used.
> 

Thanks for fixing this issue!

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thank you!

> Fixes: 581a7b26ab364 ("selftests/ftrace: Add dynamic events argument limitation test case")
> Reported-by: Mark Brown <broonie@kernel.org>
> Closes: https://lore.kernel.org/all/ccc40f2b-4b9e-4abd-8daf-d22fce2a86f0@sirena.org.uk/
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
>  .../test.d/dynevent/dynevent_limitations.tc   | 23 ++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/ftrace/test.d/dynevent/dynevent_limitations.tc b/tools/testing/selftests/ftrace/test.d/dynevent/dynevent_limitations.tc
> index 6b94b678741a..885631c02623 100644
> --- a/tools/testing/selftests/ftrace/test.d/dynevent/dynevent_limitations.tc
> +++ b/tools/testing/selftests/ftrace/test.d/dynevent/dynevent_limitations.tc
> @@ -7,11 +7,32 @@
>  MAX_ARGS=128
>  EXCEED_ARGS=$((MAX_ARGS + 1))
>  
> +# bash and dash evaluate variables differently.
> +# dash will evaluate '\\' every time it is read whereas bash does not.
> +#
> +#   TEST_STRING="$TEST_STRING \\$i"
> +#   echo $TEST_STRING
> +#
> +# With i=123
> +# On bash, that will print "\123"
> +# but on dash, that will print the escape sequence of \123 as the \ will
> +# be interpreted again in the echo.
> +#
> +# Set a variable "bs" to save a double backslash, then echo that
> +# to "ts" to see if $ts changed or not. If it changed, it's dash,
> +# if not, it's bash, and then bs can equal a single backslash.
> +bs='\\'
> +ts=`echo $bs`
> +if [ "$ts" = '\\' ]; then
> +  # this is bash
> +  bs='\'
> +fi
> +
>  check_max_args() { # event_header
>    TEST_STRING=$1
>    # Acceptable
>    for i in `seq 1 $MAX_ARGS`; do
> -    TEST_STRING="$TEST_STRING \\$i"
> +    TEST_STRING="$TEST_STRING $bs$i"
>    done
>    echo "$TEST_STRING" >> dynamic_events
>    echo > dynamic_events
> -- 
> 2.47.2
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>