[Crash-utility] Re: [PATCH v6 4/5] fix gdb_interface: restore gdb's output streams at end of gdb_interface

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(a)linux.ibm.com&gt; --- 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 ()) {