Currently for most gdb_interface call with a non-null file pointer,
GDB's output stream is replaced with the passed file pointer
'info threads', which is a gdb passthrough, fails to print all thread
after support was added to get registers from crash_target:
Id Target Id Frame
1 CPU 0 <unavailable> in ?? ()
2 CPU 1
Also, it was noticed that 'info threads' will print all threads when
'info threads' is run 2nd and subsequent times.
This incomplete output of 'info threads' was due to a subtle bug in
gdb_interface. For any passthrough, it switches the gdb output and
stderr streams to another FILE* passed in `struct gnu_request`.
But it does not restore the original FILE* stream gdb was using.
For each thread, which does not have it's registers yet, gdb goes to
crash_target::fetch_registers to ask for registers. Which in turn might
call 'datatype_info', which causes gdb's output stream to be set to
'/dev/null'
So all output after the call to datatype_info will go to /dev/null, and
hence the data to be printed is lost.
When 'info threads' is run a second time, it sets GDB's output streams
to something which is NOT /dev/null, and since it already has registers
for each thread, it does not go to 'crash_target::fetch_registers' and
hence the output stream is consistent, and we get complete output
So what happened is this:
1. GDB prints thread 1 (CPU 0) correctly since it had it's registers
cached during initialisation in 'crash_target_init' (at that times
registers where still unavailable, so it shows unavailable)
2. At CPU 1, to print the frame, it goes to 'crash_target::fetch_registers'
to get the registers
3. crash_target then calls 'machdep->get_cpu_reg' which finally ends up
at 'get_netdump_regs' which uses 'datatype_info' to get info of 'elf_prstatus.pr_reg'
4. Inside 'datatype_info', we set the file stream to /dev/null
(pc->nullfp). Then it passes this request object to gdb_interface
5. gdb_interface replaces GDB's output stream to the passed stream, ie.
/dev/null
6. Now, ALL FUTURE WRITES EVEN BY GDB, SUCH AS FOR THE 'info threads'
COMMAND GOES TO THIS STREAM gnu_request->fp, WHICH is /dev/null,
until someone else sets the non-null stream later
7. Similar behaviour occurs with all other threads
8. And hence we lose all subsequent output by GDB also
When 'info threads' is run a second time:
1. 'gdb_pass_through' passes a default FILE* to 'gdb_interface', which
is not /dev/null
2. Since the registers are cached for all threads now, it does not go
back to 'crash_target' for registers, and hence no call is made to
'datatype_info', hence GDB's output stream is not changing
3. Since the gdb's stream, even though not what it was using originally,
it's still valid, and not '/dev/null', hence we get the complete output
Fix this by restoring the original output streams, after gdb_interface
has handled the output
Signed-off-by: Aditya Gupta <adityag@xxxxxxxxxxxxx>
---
gdb-10.2.patch | 46 +++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 45 insertions(+), 1 deletion(-)
diff --git a/gdb-10.2.patch b/gdb-10.2.patch
index 8c1b43eb07b7..a4e45fa0e6fa 100644
--- a/gdb-10.2.patch
+++ b/gdb-10.2.patch
@@ -1484,13 +1484,29 @@ exit 0
+ }
+}
+#endif
+--- gdb-10.2/gdb/ui-file.c.orig
++++ gdb-10.2/gdb/ui-file.c
+@@ -161,6 +161,12 @@ stdio_file::~stdio_file ()
+ fclose (m_file);
+ }
+
++FILE*
++stdio_file::get_stream(void)
++{
++ return m_file;
++}
++
+ void
+ stdio_file::set_stream (FILE *file)
+ {
--- gdb-10.2/gdb/ui-file.h.orig
+++ gdb-10.2/gdb/ui-file.h
-@@ -195,10 +195,10 @@ class stdio_file : public ui_file
+@@ -195,10 +195,11 @@ class stdio_file : public ui_file
bool can_emit_style_escape () override;
-private:
++ FILE *get_stream(void);
/* Sets the internal stream to FILE, and saves the FILE's file
descriptor in M_FD. */
void set_stream (FILE *file);
@@ -3267,3 +3283,31 @@ exit 0
for (compunit_symtab *cust : objfile->compunits ())
{
+--- gdb-10.2/gdb/symtab.c.orig
++++ gdb-10.2/gdb/symtab.c
+@@ -6964,8 +6964,12 @@ void
+ gdb_command_funnel_1(struct gnu_request *req)
+ {
+ struct symbol *sym;
++ FILE *original_stdout_stream = nullptr;
++ FILE *original_stderr_stream = nullptr;
+
+ if (req->command != GNU_VERSION && req->command != GNU_USER_PRINT_OPTION) {
++ original_stdout_stream = (dynamic_cast< stdio_file * >gdb_stdout)->get_stream();
++ original_stderr_stream = (dynamic_cast< stdio_file * >gdb_stderr)->get_stream();
+ (dynamic_cast<stdio_file *>gdb_stdout)->set_stream(req->fp);
+ (dynamic_cast<stdio_file *>gdb_stderr)->set_stream(req->fp);
+ }
+@@ -7068,6 +7072,12 @@ gdb_command_funnel_1(struct gnu_request *req)
+ req->flags |= GNU_COMMAND_FAILED;
+ break;
+ }
++
++ /* Restore the streams gdb output was using */
++ if (original_stdout_stream)
++ (dynamic_cast<stdio_file *>gdb_stdout)->set_stream(original_stdout_stream);
++ if (original_stderr_stream)
++ (dynamic_cast<stdio_file *>gdb_stderr)->set_stream(original_stderr_stream);
+ }
+
+ /*
--
2.42.0
--
Crash-utility mailing list -- devel@xxxxxxxxxxxxxxxxxxxxxxxxxxx
To unsubscribe send an email to devel-leave@xxxxxxxxxxxxxxxxxxxxxxxxxxx
https://${domain_name}/admin/lists/devel.lists.crash-utility.osci.io/
Contribution Guidelines: https://github.com/crash-utility/crash/wiki
