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authorPedro Alves <palves@redhat.com>2014-03-05 14:18:28 +0000
committerPedro Alves <palves@redhat.com>2014-03-05 14:18:28 +0000
commit0f26cec1fd54c0428fda6c93c0375400e1bca738 (patch)
tree6d0a754bc753fd152d3ecec1b4105ecbf494a396 /gdb/dcache.h
parent4b95cf5c0c75d6efc1b2f96af72317aecca079f1 (diff)
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PR gdb/16575: stale breakpoint instructions in the code cache
In non-stop mode, or rather, breakpoints always-inserted mode, the code cache can easily end up with stale breakpoint instructions: All it takes is filling a cache line when breakpoints already exist in that memory region, and then delete the breakpoint. Vis. (from the new test): (gdb) set breakpoint always-inserted on (gdb) b 23 Breakpoint 2 at 0x400540: file ../../../src/gdb/testsuite/gdb.base/breakpoint-shadow.c, line 23. (gdb) b 24 Breakpoint 3 at 0x400547: file ../../../src/gdb/testsuite/gdb.base/breakpoint-shadow.c, line 24. disass main Dump of assembler code for function main: 0x000000000040053c <+0>: push %rbp 0x000000000040053d <+1>: mov %rsp,%rbp => 0x0000000000400540 <+4>: movl $0x1,-0x4(%rbp) 0x0000000000400547 <+11>: movl $0x2,-0x4(%rbp) 0x000000000040054e <+18>: mov $0x0,%eax 0x0000000000400553 <+23>: pop %rbp 0x0000000000400554 <+24>: retq End of assembler dump. So far so good. Now flush the code cache: (gdb) set code-cache off (gdb) set code-cache on Requesting a disassembly works as expected, breakpoint shadowing is applied: (gdb) disass main Dump of assembler code for function main: 0x000000000040053c <+0>: push %rbp 0x000000000040053d <+1>: mov %rsp,%rbp => 0x0000000000400540 <+4>: movl $0x1,-0x4(%rbp) 0x0000000000400547 <+11>: movl $0x2,-0x4(%rbp) 0x000000000040054e <+18>: mov $0x0,%eax 0x0000000000400553 <+23>: pop %rbp 0x0000000000400554 <+24>: retq End of assembler dump. However, now delete the breakpoints: (gdb) delete Delete all breakpoints? (y or n) y And disassembly shows the old breakpoint instructions: (gdb) disass main Dump of assembler code for function main: 0x000000000040053c <+0>: push %rbp 0x000000000040053d <+1>: mov %rsp,%rbp => 0x0000000000400540 <+4>: int3 0x0000000000400541 <+5>: rex.RB cld 0x0000000000400543 <+7>: add %eax,(%rax) 0x0000000000400545 <+9>: add %al,(%rax) 0x0000000000400547 <+11>: int3 0x0000000000400548 <+12>: rex.RB cld 0x000000000040054a <+14>: add (%rax),%al 0x000000000040054c <+16>: add %al,(%rax) 0x000000000040054e <+18>: mov $0x0,%eax 0x0000000000400553 <+23>: pop %rbp 0x0000000000400554 <+24>: retq End of assembler dump. Those breakpoint instructions are no longer installed in target memory they're stale in the code cache. Easily confirmed by just disabling the code cache: (gdb) set code-cache off (gdb) disass main Dump of assembler code for function main: 0x000000000040053c <+0>: push %rbp 0x000000000040053d <+1>: mov %rsp,%rbp => 0x0000000000400540 <+4>: movl $0x1,-0x4(%rbp) 0x0000000000400547 <+11>: movl $0x2,-0x4(%rbp) 0x000000000040054e <+18>: mov $0x0,%eax 0x0000000000400553 <+23>: pop %rbp 0x0000000000400554 <+24>: retq End of assembler dump. I stumbled upon this when writing a patch to infrun.c, that made handle_inferior_event & co fill in the cache before breakpoints were removed from the target. Recall that wait_for_inferior flushes the dcache for every event. So in that case, always-inserted mode was not necessary to trigger this. It's just a convenient way to expose the issue. The dcache works at the raw memory level. We need to update it whenever memory is written, no matter what kind of target memory object was originally passed down by the caller. The issue is that the dcache update code isn't reached when a caller explicitly writes raw memory. Breakpoint insertion/removal is one such case -- mem-break.c uses target_write_read_memory/target_write_raw_memory. The fix is to move the dcache update code from memory_xfer_partial_1 to raw_memory_xfer_partial so that it's always reachable. When we do that, we can actually simplify a series of things. memory_xfer_partial_1 no longer needs to handle writes for any kind of memory object, and therefore dcache_xfer_memory no longer needs to handle writes either. So the latter (dcache_xfer_memory) and its callees can be simplified to only care about reads. While we're touching dcache_xfer_memory's prototype, might as well rename it to reflect that fact that it only handles reads, and make it follow the new target_xfer_status/xfered_len style. This made me notice that dcache_xfer_memory loses the real error status if a memory read fails: we could have failed to read due to TARGET_XFER_E_UNAVAILABLE, for instance, but we always return TARGET_XFER_E_IO, hence the FIXME note. I felt that fixing that fell out of the scope of this patch. Currently dcache_xfer_memory handles the case of a write failing. The whole cache line is invalidated when that happens. However, dcache_update, the sole mechanism for handling writes that will remain after the patch, does not presently handle that scenario. That's a bug. The patch makes it handle that, by passing down the target_xfer_status status from the caller, so that it can better decide what to do itself. While I was changing the function's prototype, I constified the myaddr parameter, getting rid of the need for the cast as seen in its existing caller. Tested on x86_64 Fedora 17, native and gdbserver. gdb/ 2014-03-05 Pedro Alves <palves@redhat.com> PR gdb/16575 * dcache.c (dcache_poke_byte): Constify ptr parameter. Return void. Update comment. (dcache_xfer_memory): Delete. (dcache_read_memory_partial): New, based on the read bits of dcache_xfer_memory. (dcache_update): Add status parameter. Use ULONGEST for len, and adjust. Discard cache lines if the reason for the update was error. * dcache.h (dcache_xfer_memory): Delete declaration. (dcache_read_memory_partial): New declaration. (dcache_update): Update prototype. * target.c (raw_memory_xfer_partial): Update the dcache here. (memory_xfer_partial_1): Don't handle dcache writes here. gdb/testsuite/ 2014-03-05 Pedro Alves <palves@redhat.com> PR gdb/16575 * gdb.base/breakpoint-shadow.exp (compare_disassembly): New procedure. (top level): Adjust to use it. Add tests that exercise breakpoint interaction with the code-cache.
Diffstat (limited to 'gdb/dcache.h')
-rw-r--r--gdb/dcache.h15
1 files changed, 8 insertions, 7 deletions
diff --git a/gdb/dcache.h b/gdb/dcache.h
index 780dc30..020abd6 100644
--- a/gdb/dcache.h
+++ b/gdb/dcache.h
@@ -32,12 +32,13 @@ DCACHE *dcache_init (void);
/* Free a DCACHE. */
void dcache_free (DCACHE *);
-/* Simple to call from <remote>_xfer_memory. */
-
-int dcache_xfer_memory (struct target_ops *ops, DCACHE *cache, CORE_ADDR mem,
- gdb_byte *my, int len, int should_write);
-
-void dcache_update (DCACHE *dcache, CORE_ADDR memaddr, gdb_byte *myaddr,
- int len);
+enum target_xfer_status
+ dcache_read_memory_partial (struct target_ops *ops, DCACHE *dcache,
+ CORE_ADDR memaddr, gdb_byte *myaddr,
+ ULONGEST len, ULONGEST *xfered_len);
+
+void dcache_update (DCACHE *dcache, enum target_xfer_status status,
+ CORE_ADDR memaddr, const gdb_byte *myaddr,
+ ULONGEST len);
#endif /* DCACHE_H */