Re: [PATCH] GDB/testsuite: Fix schedlock.exp crash due to empty $after_args

[Kevin Buettner <kevinb@redhat.com>]

On Wed, 22 Apr 2026 23:21:05 +0100 (BST)
"Maciej W. Rozycki" <macro@orcam.me.uk> wrote:

> From: "Maciej W. Rozycki" <macro@orcam.me.uk>
> To: gdb-patches@sourceware.org
> Subject: [PATCH] GDB/testsuite: Fix schedlock.exp crash due to empty
> $after_args Date: Wed, 22 Apr 2026 23:21:05 +0100 (BST)
> User-Agent: Alpine 2.21 (DEB 202 2017-01-01)
> 
> Prevent gdb.threads/schedlock.exp from crashing due to a premature exit 
> of the debuggee causing an attempt to use a nil value as an arithmetic 
> operand:
> 
> [...]
> (gdb) FAIL: gdb.threads/schedlock.exp: schedlock=off: cmd=next:
> call_function=0: next to increment, 9 bt
> The current thread has terminated
> (gdb) FAIL: gdb.threads/schedlock.exp: schedlock=off: cmd=next:
> call_function=0: find current thread, after FAIL:
> gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=0: next
> does not change thread (switched to thread ) print args Cannot access
> memory at address 0x410ab0 (gdb) FAIL: gdb.threads/schedlock.exp:
> schedlock=off: cmd=next: call_function=0: listed args, after ERROR: tcl
> error sourcing .../gdb/testsuite/gdb.threads/schedlock.exp. ERROR: can't
> use empty string as operand of "-" while executing
> "if {$cmd == "continue"
> 		    || [lindex $before_args $i] == [lindex $after_args
> $i] - 10} { pass "$test"
> 		} else {
> 		    fail "$test (wrong amo..."
>     (procedure "check_result" line 31)
>     invoked from within
> "check_result $cmd $curthread $before_args $locked"
>     (procedure "test_step" line 26)
>     invoked from within
> "test_step $schedlock "next" $call_function"
>     ("uplevel" body line 2)
>     invoked from within
> "uplevel 1 $body"
>     invoked from within
> "with_test_prefix "call_function=$call_function" {
> 		    test_step $schedlock "next" $call_function
> 		}"
>     ("foreach" body line 2)
>     invoked from within
> "foreach call_function {0 1} {
> 		with_test_prefix "call_function=$call_function" {
> 		    test_step $schedlock "next" $call_function
> 		}
> 	    }"
>     ("uplevel" body line 6)
>     invoked from within
> "uplevel 1 $body"
>     invoked from within
> "with_test_prefix "cmd=next" {
> 	    # In GDB <= 7.9, with schedlock "step", "next" would
> 	    # unlock threads when stepping over a function call.  Thi..."
>     ("uplevel" body line 5)
>     invoked from within
> "uplevel 1 $body"
>     invoked from within
> "with_test_prefix "schedlock=$schedlock" {
> 	with_test_prefix "cmd=step" {
> 	    test_step $schedlock "step" 0
> 	}
> 	with_test_prefix "cmd=next" {
> 	    # I..."
>     ("foreach" body line 2)
>     invoked from within
> "foreach schedlock {"off" "step" "on"} {
>     with_test_prefix "schedlock=$schedlock" {
> 	with_test_prefix "cmd=step" {
> 	    test_step $schedlock "step" ..."
>     (file ".../gdb/testsuite/gdb.threads/schedlock.exp" line 297)
>     invoked from within
> "source .../gdb/testsuite/gdb.threads/schedlock.exp"
>     ("uplevel" body line 1)
>     invoked from within
> "uplevel #0 source .../gdb/testsuite/gdb.threads/schedlock.exp"
>     invoked from within
> "catch "uplevel #0 source $test_file_name""
> Remote debugging from host xx.xx.xx.xx, port 56596
> monitor exit
> (gdb) Killing process(es): 22658
> testcase .../gdb/testsuite/gdb.threads/schedlock.exp completed in 32
> seconds
> 
> Here `print args' has failed to produce output matching the pattern 
> expected by `get_args' and consequently an empty value has been assigned 
> to $after_args.  Subsequently a calculation is attempted on an element 
> of said value treated as a list: `[lindex $after_args $i] - 10' and that 
> has caused the crash because the resulting minuend is nil.
> 
> There are various expressions $after_args and other variables set from 
> the result of `get_args' are used in, however the majority are equality 
> operations, which succeed producing a result even where a nil operand is 
> involved.  Given that this is a test failure scenario anyway follow the 
> path of least resistance, ignore the other expressions and just prevent 
> the crash from triggering here by checking for an attempt to retrieve an 
> inexistent element of $after_args for this calculation, and report it as 
> a test failure outright letting the script proceed:
> 
> [...]
> (gdb) FAIL: gdb.threads/schedlock.exp: schedlock=off: cmd=next:
> call_function=0: next to increment, 9 bt
> The current thread has terminated
> (gdb) FAIL: gdb.threads/schedlock.exp: schedlock=off: cmd=next:
> call_function=0: find current thread, after FAIL:
> gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=0: next
> does not change thread (switched to thread ) print args Cannot access
> memory at address 0x410ab0 (gdb) FAIL: gdb.threads/schedlock.exp:
> schedlock=off: cmd=next: call_function=0: listed args, after FAIL:
> gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=0:
> current thread advanced - unlocked (no arg #1) PASS:
> gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=0:
> other threads ran - unlocked set scheduler-locking off (gdb) PASS:
> gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=1: set
> scheduler-locking off [...] --- gdb/testsuite/gdb.threads/schedlock.exp |
>    8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
> 
> gdb-test-schedlock-no-args.diff
> Index: binutils-gdb/gdb/testsuite/gdb.threads/schedlock.exp
> ===================================================================
> --- binutils-gdb.orig/gdb/testsuite/gdb.threads/schedlock.exp
> +++ binutils-gdb/gdb/testsuite/gdb.threads/schedlock.exp
> @@ -232,8 +232,12 @@ proc check_result { cmd before_thread be
>  	    }
>  	} else {
>  	    if {$i == $before_thread} {
> -		if {$cmd == "continue"
> -		    || [lindex $before_args $i] == [lindex $after_args
> $i] - 10} {
> +		if {$cmd == "continue"} {
> +		    pass "$test"
> +		} elseif {[llength $after_args] <= $i} {
> +		    fail "$test (no arg #$i)"
> +		} elseif {[lindex $before_args $i] \
> +			  == [lindex $after_args $i] - 10} {
>  		    pass "$test"
>  		} else {
>  		    fail "$test (wrong amount)"
> 

LGTM.  Thanks for this fix.

Approved-by: Kevin Buettner <kevinb@redhat.com>