[AArch64] MTE corefile support

Teach GDB how to dump memory tags for AArch64 when using the gcore command
and how to read memory tag data back from a core file generated by GDB
(via gcore) or by the Linux kernel.

The format is documented in the Linux Kernel documentation [1].

Each tagged memory range (listed in /proc/<pid>/smaps) gets dumped to its
own PT_AARCH64_MEMTAG_MTE segment. A section named ".memtag" is created for each
of those segments when reading the core file back.

To save a little bit of space, given MTE tags only take 4 bits, the memory tags
are stored packed as 2 tags per byte.

When reading the data back, the tags are unpacked.

I've added a new testcase to exercise the feature.

Build-tested with --enable-targets=all and regression tested on aarch64-linux
Ubuntu 20.04.

[1] Documentation/arm64/memory-tagging-extension.rst (Core Dump Support)

1 files changed, 19 insertions, 0 deletions

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 1c017e4..382df00 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -25765,6 +25765,25 @@ options that can be controlled at runtime and emulates the @code{prctl}
 option @code{PR_SET_TAGGED_ADDR_CTRL}.  For further information, see the
 documentation in the Linux kernel.
 
+@value{GDBN} supports dumping memory tag data to core files through the
+@command{gcore} command and reading memory tag data from core files generated
+by the @command{gcore} command or the Linux kernel.
+
+When a process uses memory-mapped pages protected by memory tags (for
+example, AArch64 MTE), this additional information will be recorded in
+the core file in the event of a crash or if @value{GDBN} generates a core file
+from the current process state.
+
+The memory tag data will be used so developers can display the memory
+tags from a particular memory region (using the @samp{m} modifier to the
+@command{x} command, using the @command{print} command or using the various
+@command{memory-tag} subcommands.
+
+In the case of a crash, @value{GDBN} will attempt to retrieve the memory tag
+information automatically from the core file, and will show one of the above
+messages depending on whether the synchronous or asynchronous mode is selected.
+@xref{Memory Tagging}. @xref{Memory}.
+
 @node i386
 @subsection x86 Architecture-specific Issues