[PATCH 0/7] RFC Memory tagging support

H.J. Lu hjl.tools@gmail.com
Mon Jun 15 15:03:20 GMT 2020

On Mon, Jun 15, 2020 at 7:43 AM Richard Earnshaw <rearnsha@arm.com> wrote:
> Last year I posted a preliminary set of patches for discussion
> purposes on adding support for tagged memory to glibc.  This version
> polishes that to the point where I believe it is now deployable.
> The first four patches are generic changes, the final three add the
> aarch64 specific code.
> The first patch simply adds a new configuration option to the build
> system which can be turned on with the option --enable-memory-tagging.
> The default at present is 'no'.
> The second patch adds a glibc tunable that can be used at run-time to
> enable the feature (the default again, is disabled).  This tunable
> would be always present, but have no effect on systems lacking support
> for memory tagging.  I've structured the tunable as a bit-mask of
> features that can be used with memory tagging, though at present only
> two bits have defined uses.
> The third patch is the meat of the changes; it adds the changes to the
> malloc APIs.  I've tried as far as possible to ensure that when memory
> tagging is disabled, there is no change in behaviour, even when the
> memory tagging is configured into the library, but there are
> inevitably a small number of changes needed in the optimizations that
> calloc performs since tagging would require that all the tags were
> correctly set, even if the memory does not strictly have to be zeroed.
> I've made use of function pointers in the code, much the same way as
> the morecore hook is used, so that when tagging is disabled, the
> functions called are the same as the traditional operations; this also
> ensures that glibc does not require any internal ifunc resolution in
> order to work.
> The fourth patch adds support for the new prctl operations that are
> being proposed to the linux kernel.  The kernel changes are to a
> generic header and this patch mirrors that design decision in glibc.
> The fifth patch is a place-holder, so that this series of changes is
> stand-alone.  Work is already underway to change the string operations
> to be MTE safe without losing too much in the way of performance.  I
> expect this patch to be removed entirely before the series is
> committed.
> The final two patches add the remaining aarch64 support.  The first
> adds the support code to examine the tunable and HW caps; and enable
> memory tagging in the kernel as needed.  The second adds the final
> pieces needed to support memory tagging in glibc.

Obviously, pointer comparison and algorithm will be impacted by MTE.
>From what you are proposing, only parts of glibc will be MTE compatible.
Is this correct?


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