From FEDORA_PATCHES Mon Sep 17 00:00:00 2001

From: Keith Seitz <keiths@redhat.com>

Date: Mon, 27 Jul 2020 17:11:49 -0400

Subject: gdb-rhbz1842691-corefile-mem-access-4of15.patch

;; Provide access to non SEC_HAS_CONTENTS core file sections

;; Kevin Buettner, RH BZ 1842961

   Author: Kevin Buettner <kevinb@redhat.com>

   Date:   Wed Mar 4 17:42:42 2020 -0700

    Provide access to non SEC_HAS_CONTENTS core file sections

    Consider the following program:

    - - - mkmmapcore.c - - -

    static char *buf;

    int

    main (int argc, char **argv)

    {

      buf = mmap (NULL, 8192, PROT_READ | PROT_WRITE,

                  MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);

      abort ();

    }

    - - - end mkmmapcore.c - - -

    Compile it like this:

    gcc -g -o mkmmapcore mkmmapcore.c

    Now let's run it from GDB.  I've already placed a breakpoint on the

    line with the abort() call and have run to that breakpoint.

    Breakpoint 1, main (argc=1, argv=0x7fffffffd678) at mkmmapcore.c:11

    11	  abort ();

    (gdb) x/x buf

    0x7ffff7fcb000:	0x00000000

    Note that we can examine the memory allocated via the call to mmap().

    Now let's try debugging a core file created by running this program.

    Depending on your system, in order to make a core file, you may have to

    run the following as root (or using sudo):

        echo core > /proc/sys/kernel/core_pattern

    It may also be necessary to do:

        ulimit -c unlimited

    I'm using Fedora 31. YMMV if you're using one of the BSDs or some other

    (non-Linux) system.

    This is what things look like when we debug the core file:

        [kev@f31-1 tmp]$ gdb -q ./mkmmapcore core.304767

        Reading symbols from ./mkmmapcore...

        [New LWP 304767]

        Core was generated by `/tmp/mkmmapcore'.

        Program terminated with signal SIGABRT, Aborted.

        #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50

        50	  return ret;

        (gdb) x/x buf

        0x7ffff7fcb000:	Cannot access memory at address 0x7ffff7fcb000

    Note that we can no longer access the memory region allocated by mmap().

    Back in 2007, a hack for GDB was added to _bfd_elf_make_section_from_phdr()

    in bfd/elf.c:

    	  /* Hack for gdb.  Segments that have not been modified do

    	     not have their contents written to a core file, on the

    	     assumption that a debugger can find the contents in the

    	     executable.  We flag this case by setting the fake

    	     section size to zero.  Note that "real" bss sections will

    	     always have their contents dumped to the core file.  */

    	  if (bfd_get_format (abfd) == bfd_core)

    	    newsect->size = 0;

    You can find the entire patch plus links to other discussion starting

    here:

        https://sourceware.org/ml/binutils/2007-08/msg00047.html

    This hack sets the size of certain BFD sections to 0, which

    effectively causes GDB to ignore them.  I think it's likely that the

    bug described above existed even before this hack was added, but I

    have no easy way to test this now.

    The output from objdump -h shows the result of this hack:

     25 load13        00000000  00007ffff7fcb000  0000000000000000  00013000  2**12

                      ALLOC

    (The first field, after load13, shows the size of 0.)

    Once the hack is removed, the output from objdump -h shows the correct

    size:

     25 load13        00002000  00007ffff7fcb000  0000000000000000  00013000  2**12

                      ALLOC

    (This is a digression, but I think it's good that objdump will now show

    the correct size.)

    If we remove the hack from bfd/elf.c, but do nothing to GDB, we'll

    see the following regression:

    FAIL: gdb.base/corefile.exp: print coremaker_ro

    The reason for this is that all sections which have the BFD flag

    SEC_ALLOC set, but for which SEC_HAS_CONTENTS is not set no longer

    have zero size.  Some of these sections have data that can (and should)

    be read from the executable.  (Sections for which SEC_HAS_CONTENTS

    is set should be read from the core file; sections which do not have

    this flag set need to either be read from the executable or, failing

    that, from the core file using whatever BFD decides is the best value

    to present to the user - it uses zeros.)

    At present, due to the way that the target strata are traversed when

    attempting to access memory, the non-SEC_HAS_CONTENTS sections will be

    read as zeroes from the process_stratum (which in this case is the

    core file stratum) without first checking the file stratum, which is

    where the data might actually be found.

    What we should be doing is this:

    - Attempt to access core file data for SEC_HAS_CONTENTS sections.

    - Attempt to access executable file data if the above fails.

    - Attempt to access core file data for non SEC_HAS_CONTENTS sections, if

      both of the above fail.

    This corresponds to the analysis of Daniel Jacobowitz back in 2007

    when the hack was added to BFD:

        https://sourceware.org/legacy-ml/binutils/2007-08/msg00045.html

    The difference, observed by Pedro in his review of my v1 patches, is

    that I'm using "the section flags as proxy for the p_filesz/p_memsz

    checks."

    gdb/ChangeLog:

    	PR corefiles/25631

    	* corelow.c (core_target:xfer_partial):  Revise

    	TARGET_OBJECT_MEMORY case to consider non-SEC_HAS_CONTENTS

    	case after first checking the stratum beneath the core

    	target.

    	(has_all_memory): Return true.

    	* target.c (raw_memory_xfer_partial): Revise comment

    	regarding use of has_all_memory.

diff --git a/gdb/corelow.c b/gdb/corelow.c

--- a/gdb/corelow.c

+++ b/gdb/corelow.c

@@ -93,7 +93,7 @@ public:

   const char *thread_name (struct thread_info *) override;

-  bool has_all_memory () override { return false; }

+  bool has_all_memory () override { return true; }

   bool has_memory () override;

   bool has_stack () override;

   bool has_registers () override;

@@ -754,12 +754,47 @@ core_target::xfer_partial (enum target_object object, const char *annex,

   switch (object)

     {

     case TARGET_OBJECT_MEMORY:

-      return (section_table_xfer_memory_partial

-	      (readbuf, writebuf,

-	       offset, len, xfered_len,

-	       m_core_section_table.sections,

-	       m_core_section_table.sections_end));

+      {

+	enum target_xfer_status xfer_status;

+

+	/* Try accessing memory contents from core file data,

+	   restricting consideration to those sections for which

+	   the BFD section flag SEC_HAS_CONTENTS is set.  */

+	auto has_contents_cb = [] (const struct target_section *s)

+	  {

+	    return ((s->the_bfd_section->flags & SEC_HAS_CONTENTS) != 0);

+	  };

+	xfer_status = section_table_xfer_memory_partial

+			(readbuf, writebuf,

+			 offset, len, xfered_len,

+			 m_core_section_table.sections,

+			 m_core_section_table.sections_end,

+			 has_contents_cb);

+	if (xfer_status == TARGET_XFER_OK)

+	  return TARGET_XFER_OK;

+

+	/* Now check the stratum beneath us; this should be file_stratum.  */

+	xfer_status = this->beneath ()->xfer_partial (object, annex, readbuf,

+						      writebuf, offset, len,

+						      xfered_len);

+	if (xfer_status == TARGET_XFER_OK)

+	  return TARGET_XFER_OK;

+	/* Finally, attempt to access data in core file sections with

+	   no contents.  These will typically read as all zero.  */

+	auto no_contents_cb = [&] (const struct target_section *s)

+	  {

+	    return !has_contents_cb (s);

+	  };

+	xfer_status = section_table_xfer_memory_partial

+			(readbuf, writebuf,

+			 offset, len, xfered_len,

+			 m_core_section_table.sections,

+			 m_core_section_table.sections_end,

+			 no_contents_cb);

+

+	return xfer_status;

+      }

     case TARGET_OBJECT_AUXV:

       if (readbuf)

 	{

diff --git a/gdb/target.c b/gdb/target.c

--- a/gdb/target.c

+++ b/gdb/target.c

@@ -967,8 +967,11 @@ raw_memory_xfer_partial (struct target_ops *ops, gdb_byte *readbuf,

       if (res == TARGET_XFER_UNAVAILABLE)

 	break;

-      /* We want to continue past core files to executables, but not

-	 past a running target's memory.  */

+      /* Don't continue past targets which have all the memory.

+         At one time, this code was necessary to read data from

+	 executables / shared libraries when data for the requested

+	 addresses weren't available in the core file.  But now the

+	 core target handles this case itself.  */

       if (ops->has_all_memory ())

 	break;