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authorJoel Brobecker <brobecker@adacore.com>2014-11-20 20:41:25 +0400
committerJoel Brobecker <brobecker@adacore.com>2014-11-23 14:34:52 +0400
commite8af5d7a5cd4c58136a4733e87612f49061bf28b (patch)
tree5de0dd2e5c83efb5e9200b4c6136e44fcc403b97 /bfd
parenta344fc094daa257557786eb2ce871debf38456ba (diff)
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Always consider infcall breakpoints as non-permanent.
A recent change... commit 1a853c5224e2b8fedfac6d029365522b83080b40 Date: Wed Nov 12 10:10:49 2014 +0000 Subject: make "permanent breakpoints" per location and disableable ... broke function calls on sparc-elf when running over QEMU. Any function call should demonstrate the problem. For instance, seen from the debugger: (gdb) call pn(1234) [Inferior 1 (Remote target) exited normally] The program being debugged exited while in a function called from GDB. Evaluation of the expression containing the function And seen from QEMU: qemu: fatal: Trap 0x02 while interrupts disabled, Error state [register dump removed] What happens in this case is that GDB sets the inferior function call by not only creating the dummy frame, but also writing a breakpoint instruction at the return address for our function call. See infcall.c: /* Write a legitimate instruction at the point where the infcall breakpoint is going to be inserted. While this instruction is never going to be executed, a user investigating the memory from GDB would see this instruction instead of random uninitialized bytes. We chose the breakpoint instruction as it may look as the most logical one to the user and also valgrind 3.7.0 needs it for proper vgdb inferior calls. If software breakpoints are unsupported for this target we leave the user visible memory content uninitialized. */ bp_addr_as_address = bp_addr; bp_bytes = gdbarch_breakpoint_from_pc (gdbarch, &bp_addr_as_address, &bp_size); if (bp_bytes != NULL) write_memory (bp_addr_as_address, bp_bytes, bp_size); This instruction triggers a change introduced by the commit above, where we consider bp locations as being permanent breakpoints if there is already a breakpoint instruction at that address: + if (bp_loc_is_permanent (loc)) + { + loc->inserted = 1; + loc->permanent = 1; + } As a result, when resuming the program's execution for the inferior function call, GDB decides that it does not need to insert a breakpoint at this address, expecting the target to just report a SIGTRAP when trying to execute that instruction. But unfortunately for us, at least some versions of QEMU for SPARC just terminate the execution entirely instead of reporting a breakpoint, thus producing the behavior reported here. Although it appears like QEMU might be misbehaving and should therefore be fixed (to be verified) from the user's point of view, the recent change does introduce a regression. So this patch tries to mitigate a bit the damage by handling such infcall breakpoints as special and making sure that they are never considered permanent, thus restoring the previous behavior specifically for those breakpoints. The option of not writing the breakpoint instructions in the first place was considered, and would probably work also. But the comment associated to it seems to indicate that there is still reason to keep it. gdb/ChangeLog: * breakpoint.c (bp_loc_is_permanent): Return 0 if LOC corresponds to a bp_call_dummy breakpoint type. Tested on x86_64-linux. Also testing on sparc-elf/QEMU using AdaCore's testsuite.
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