diff mbox

Remove breakpoint step-over information if failed to resume

Message ID alpine.LFD.2.21.1910301336270.2438@redsun52.ssa.fujisawa.hgst.com
State Superseded
Headers show

Commit Message

Maciej W. Rozycki Oct. 30, 2019, 2:26 p.m. UTC
Complement commit 31e77af205cf ("PR breakpoints/7143 - Watchpoint does 
not trigger when first set") and commit 71d378ae60a4 
("gdb.base/breakpoint-in-ro-region.exp regression on sss targets (PR 
gdb/22583)") and also remove breakpoint step-over information if the 
inferior has failed to resume, which may be due to for example a failure 
to insert a software breakpoint at an inaccessible location.  If this 
happens, the state of GDB becomes inconsistent and further attempts to 
start execution hang.

This was observed with a `riscv64-linux-gnu' cross-GDB and RISC-V/Linux 
`gdbserver' being developed and an attempt to step over the `ret' aka 
`c.jr ra' instruction where the value held in the `ra' aka `x1' register 
and therefore the address of a software-stepping breakpoint to insert is 
0.  Here's a record of a remote debug session leading to the issue:

Sending packet: $vCont?#49...Packet received: vCont;c;C;t
Packet vCont (verbose-resume) is supported
Sending packet: $vCont;c:p23cb.-1#08...Packet received: T05swbreak:;02:20da080000000000;20:b807565515000000;thread:p23cb.23cb;core:3;
Sending packet: $qXfer:libraries-svr4:read::0,1000#20...Packet received: l<library-list-svr4 version="1.0" main-lm="0x155556f160"><library name=".../sysroot/lib/ld-linux-riscv64-lp64d.so.1" lm="0x155556ea18" l_addr="0x1555556000" l_ld="0x155556de90"/><library name="linux-vdso.so.1" lm="0x155556f6f0" l_addr="0x1555570000" l_ld="0x1555570350"/></library-list-svr4>
Sending packet: $X15555607b8,2:\202\200#2e...Packet received: OK
Sending packet: $m15555607b8,2#07...Packet received: 8280
Sending packet: $m15555607b8,2#07...Packet received: 8280
Sending packet: $g#67...Packet received: 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[552 bytes omitted]
Sending packet: $m0,40#2d...Packet received: E01
Sending packet: $m0,1#fa...Packet received: E01
Sending packet: $m0,40#2d...Packet received: E01
Sending packet: $m0,1#fa...Packet received: E01
Cannot access memory at address 0x0
(gdb) stepi
Sending packet: $m15555607b8,4#09...Packet received: 82802a87
Sending packet: $m15555607b4,4#05...Packet received: 01452dbd
^Cremote_pass_ctrlc called
remote_interrupt called
^Cremote_pass_ctrlc called
Interrupted while waiting for the program.
Give up waiting? (y or n) y
Quit
(gdb) 

As observed here the `stepi' command does not even attempt to reinsert 
the breakpoint, let alone resume execution.

Correct the issue by making a call to clear global breakpoint step-over 
from the exception catch clause in `resume'.

With the change applied further `stepi' commands correctly retry 
breakpoint insertion:

(gdb) stepi
Sending packet: $m15555607b8,4#09...Packet received: 82802a87
Sending packet: $m15555607b4,4#05...Packet received: 01452dbd
Sending packet: $m15555607b8,2#07...Packet received: 8280
Sending packet: $m15555607b8,2#07...Packet received: 8280
Sending packet: $m0,40#2d...Packet received: E01
Sending packet: $m0,1#fa...Packet received: E01
Sending packet: $m0,40#2d...Packet received: E01
Sending packet: $m0,1#fa...Packet received: E01
Cannot access memory at address 0x0
(gdb)

and changing the value of the `pc' register so as not to point at a 
`ret' instruction allows execution to be actually resumed.

	gdb/
	* infrun.c (resume): Also call `clear_step_over_info' in the 
	`catch' clause.
---
Hi,

 NB, this complements rather than replacing the ("Remove stale breakpoint 
step-over information") proposed change recorded here: 
<https://sourceware.org/ml/gdb-patches/2019-10/msg00659.html>; we still 
need to clean the breakpoint step-over information if single-stepping did 
proceed as requested, but then hung and was forcibly interrupted (as it 
could happen if say an RSP JTAG probe being talked to stopped responding).

 For completeness this change was natively regression-tested with the 
`x86_64-linux-gnu' configuration, although I'd consider it obviously 
harmless and only relevant to the affected scenarios.  I think it would be 
good to have such a scenario covered in our test suite, however this is 
highly target-specific.  I'll try to think about a test case, however it 
may not be right away that I come up with one.

 Meanwhile, OK to apply?

  Maciej
---
 gdb/infrun.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

gdb-resume-clear-step-over-info.diff
diff mbox

Patch

Index: binutils-gdb/gdb/infrun.c
===================================================================
--- binutils-gdb.orig/gdb/infrun.c
+++ binutils-gdb/gdb/infrun.c
@@ -2617,7 +2617,10 @@  resume (gdb_signal sig)
 	 single-step breakpoints perturbing other threads, in case
 	 we're running in non-stop mode.  */
       if (inferior_ptid != null_ptid)
-	delete_single_step_breakpoints (inferior_thread ());
+	{
+	  delete_single_step_breakpoints (inferior_thread ());
+	  clear_step_over_info ();
+	}
       throw;
     }
 }