vdso handling

Metzger, Markus T
2014-03-10T13:05:04Z

Hello,

My name is Markus. I work for Intel on GDB in the area of
hardware-supported execution recording.

I noticed that the BFD created for the VDSO (system-provided in-memory
DSO) does not contain any BFD sections. Is this intentional? Or has
there just been no need for them?

If it just hasn't been done, yet, how would I best approach this?

Here's the problem I am trying to solve with this. May as well be
that I'm on the wrong track...

When using the btrace record target, GDB only allows access to
read-only memory during replay. The btrace record target does not
trace data so read-write memory corresponds to the end of the trace,
not the current replay position.

The implementation uses the same check that is also used for
'trust-readonly-sections", i.e.

section = target_section_by_addr (ops, offset);
if (section != NULL)
{
/* Check if the section we found is readonly. */
if ((bfd_get_section_flags (section->the_bfd_section->owner,
section->the_bfd_section)
& SEC_READONLY) != 0)

For the vdso, there is no target section, so the check fails. This
prevents GDB from disassembling vdso instructions during replay.

The vdso is processed in symbol_file_add_from_memory at
gdb/symfile-mem.c:84. It calls bfd_from_remote_memory to create a BFD
for the vdso and then processes it. If there were BFD sections, the
following small patch should add the respective target sections to GDB.

diff --git a/gdb/symfile-mem.c b/gdb/symfile-mem.c
index e3230de..cf4da38 100644
--- a/gdb/symfile-mem.c
+++ b/gdb/symfile-mem.c
@@ -91,6 +91,7 @@ symbol_file_add_from_memory (struct bfd *templ, CORE_ADDR addr, char *name,
struct section_addr_info *sai;
unsigned int i;
struct cleanup *cleanup;
+ struct target_section *sections, *sections_end, *tsec;

if (bfd_get_flavour (templ) != bfd_target_elf_flavour)
error (_("add-symbol-file-from-memory not supported for this target"));
@@ -113,6 +114,22 @@ symbol_file_add_from_memory (struct bfd *templ, CORE_ADDR addr, char *name,
error (_("Got object file from memory but can't read symbols: %s."),
bfd_errmsg (bfd_get_error ()));

+ /* Add target sections for this bfd. */
+ sections = NULL;
+ sections_end = NULL;
+ if (build_section_table (nbfd, &sections, &sections_end))
+ error (_("Failed to build section table"));
+
+ /* Adjust the target section addresses by the load address. */
+ for (tsec = sections; tsec != sections_end; ++tsec)
+ {
+ tsec->addr += loadbase;
+ tsec->endaddr += loadbase;
+ }
+
+ add_target_sections (&nbfd, sections, sections_end);
+ xfree (sections);
+
sai = alloc_section_addr_info (bfd_count_sections (nbfd));
make_cleanup (xfree, sai);
i = 0;

This should allow me to pass the above read-only section check and
allow GDB to disassemble vdso instructions.

thanks,
markus.

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