Re: [PATCH v5 15/25] AArch64: Implement the memory tagging gdbarch hooks

Luis Machado via Gdb-patches
2021-02-05T14:05:49Z

On 2/5/21 1:09 AM, Simon Marchi wrote:
>
>
> On 2021-01-27 3:21 p.m., Luis Machado via Gdb-patches wrote:
>> Updates on v4:
>>
>> - Update function names to be more scoped.
>> - Use of gdb::optional values.
>> - Fixed up gdbarch hooks.
>>
>> Updates on v2:
>>
>> - Update target methods to contain a tag type parameter.
>>
>> --
>>
>> This patch implements the memory tagging gdbarch hooks for AArch64, for
>> the MTE feature.
>>
>> gdb/ChangeLog:
>>
>> YYYY-MM-DD Luis Machado <luis.machado@linaro.org>
>>
>> * aarch64-linux-tdep.c: Include target.h, arch-utils.h, value.h.
>> (aarch64_mte_get_atag, aarch64_linux_tagged_address_p)
>> (aarch64_linux_memtag_mismatch_p, aarch64_linux_set_memtags)
>> (aarch64_linux_get_memtag, aarch64_linux_memtag_to_string): New
>> functions.
>> (aarch64_linux_init_abi): Initialize MTE-related gdbarch hooks.
>> * arch/aarch64-mte-linux.c (aarch64_mte_make_ltag_bits)
>> (aarch64_mte_make_ltag, aarch64_linux_set_ltag)
>> (aarch64_linux_get_ltag): New functions.
>> * arch/aarch64-mte-linux.h (AARCH64_MTE_LOGICAL_TAG_START_BIT)
>> (AARCH64_MTE_LOGICAL_MAX_VALUE): Define.
>> (aarch64_mte_make_ltag_bits, aarch64_mte_make_ltag)
>> (aarch64_mte_set_ltag, aarch64_mte_get_ltag): New prototypes.
>> ---
>> gdb/aarch64-linux-tdep.c | 210 +++++++++++++++++++++++++++++++++++
>> gdb/arch/aarch64-mte-linux.c | 41 ++++++-
>> gdb/arch/aarch64-mte-linux.h | 19 ++++
>> 3 files changed, 269 insertions(+), 1 deletion(-)
>>
>> diff --git a/gdb/aarch64-linux-tdep.c b/gdb/aarch64-linux-tdep.c
>> index f113d1ea30..2fe88cf016 100644
>> --- a/gdb/aarch64-linux-tdep.c
>> +++ b/gdb/aarch64-linux-tdep.c
>> @@ -30,6 +30,7 @@
>> #include "symtab.h"
>> #include "tramp-frame.h"
>> #include "trad-frame.h"
>> +#include "target.h"
>> #include "target/target.h"
>>
>> #include "regcache.h"
>> @@ -46,6 +47,9 @@
>>
>> #include "arch/aarch64-mte-linux.h"
>>
>> +#include "arch-utils.h"
>> +#include "value.h"
>> +
>> /* Signal frame handling.
>>
>> +------------+ ^
>> @@ -1513,6 +1517,187 @@ aarch64_linux_gcc_target_options (struct gdbarch *gdbarch)
>> return {};
>> }
>>
>> +/* Helper to get the allocation tag from a 64-bit ADDRESS.
>> +
>> + Return the allocation tag if successful and nullopt otherwise. */
>> +
>> +static gdb::optional<CORE_ADDR>
>> +aarch64_mte_get_atag (CORE_ADDR address)
>> +{
>> + gdb::byte_vector tags;
>> +
>> + /* Attempt to fetch the allocation tag. */
>> + if (!target_fetch_memtags (address, 0, tags, tag_allocation))
>> + return {};
>> +
>> + /* Only one tag should've been returned. Make sure we got exactly that. */
>> + gdb_assert (tags.size () == 1);
>
> The tags are returned by the target, possibly the remote target. That
> means that you could make GDB assert by returning it a memtag packet
> with the wrong number of bytes. And we don't want it to be possible to
> make GDB assert when feeding it bad input.
>
> I'd probably use error() here just for this reason.
>

Right. That makes sense. I turned this into...

if (tags.size () != 1)
error (_("Target returned an unexpected number of tags."));

>> +/* Implement the set_memtags gdbarch method. */
>> +
>> +static bool
>> +aarch64_linux_set_memtags (struct gdbarch *gdbarch, struct value *address,
>> + size_t length, const gdb::byte_vector &tags,
>> + enum memtag_type tag_type)
>> +{
>> + if (tags.empty ())
>> + return true;
>
> Can this can really happen?
>

In theory it could, if someone called gdbarch_set_memtags with an empty
vector of tag bytes. Current code won't run into this.

So this is more of a sanity check. Might as well error out on this,
instead of silently succeeding.

>> +
>> + gdb_assert (address != nullptr);
>> +
>> + CORE_ADDR addr = value_as_address (address);
>> +
>> + /* Set the logical tag or the allocation tag. */
>> + if (tag_type == tag_logical)
>> + {
>> + /* When setting logical tags, we don't care about the length, since
>> + we are only setting a single logical tag. */
>> + addr = aarch64_mte_set_ltag (addr, tags[0]);
>> +
>> + /* Update the value's content with the tag. */
>> + enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
>> + gdb_byte *srcbuf = value_contents_raw (address);
>> + store_unsigned_integer (srcbuf, sizeof (addr), byte_order, addr);
>> + }
>> + else
>> + {
>> + /* Make sure we are dealing with a tagged address to begin with. */
>> + if (!aarch64_linux_tagged_address_p (gdbarch, address))
>> + return false;
>> +
>> + /* With G being the number of tag granules and N the number of tags
>> + passed in, we can have the following cases:
>> +
>> + 1 - G == N: Store all the N tags to memory.
>> +
>> + 2 - G < N : Warn about having more tags than granules, but write G
>> + tags.
>> +
>> + 3 - G > N : This is a "fill tags" operation. We should use the tags
>> + as a pattern to fill the granules repeatedly until we have
>> + written G tags to memory.
>> + */
>> +
>> + size_t g = aarch64_mte_get_tag_granules (addr, length,
>> + AARCH64_MTE_GRANULE_SIZE);
>> + size_t n = tags.size ();
>> +
>> + if (g < n)
>> + warning (_("Got more tags than memory granules. Tags will be "
>> + "truncated."));
>> + else if (g > n)
>> + warning (_("Using tag pattern to fill memory range."));
>> +
>> + if (!target_store_memtags (addr, length, tags, tag_allocation))
>> + return false;
>> + }
>> + return true;
>
> I just realized that the gdbarch methods are overloaded, as they are
> used to both set/get the logical tag of the pointer and set/get the
> allocation tags of the allocated memory. I think it would be easier to
> understand to have four methods, get/set the local tags and get/set the
> allocation tags.

I can do a follow-up splitting the code into separate functions. As more
tags are introduced (potentially), this will eventually be needed.

>
> The documentation in gdbarch.sh for get_memtag also only talks about the
> logical tag (extracting the tag from the address).

That is an oversight. Addressed now.

>
> Since this is internal stuff, I suggest we leave this patch as-is to
> avoid some more churn, but please consider addressing it as a follow-up
> patch to the series, if you agree.
>
>> +}
>> +
>> +/* Implement the get_memtag gdbarch method. */
>> +
>> +static struct value *
>> +aarch64_linux_get_memtag (struct gdbarch *gdbarch, struct value *address,
>> + enum memtag_type tag_type)
>> +{
>> + gdb_assert (address != nullptr);
>> +
>> + CORE_ADDR addr = value_as_address (address);
>> + CORE_ADDR tag = 0;
>> +
>> + /* Get the logical tag or the allocation tag. */
>> + if (tag_type == tag_logical)
>> + tag = aarch64_mte_get_ltag (addr);
>> + else
>> + {
>> + /* Make sure we are dealing with a tagged address to begin with. */
>> + if (!aarch64_linux_tagged_address_p (gdbarch, address))
>> + return nullptr;
>> +
>> + gdb::optional<CORE_ADDR> atag = aarch64_mte_get_atag (addr);
>> +
>> + if (!atag.has_value ())
>> + return nullptr;
>> +
>> + tag = *atag;
>> + }
>> +
>> + /* Convert the tag to a value. */
>> + return value_from_ulongest (builtin_type (gdbarch)->builtin_unsigned_int,
>> + tag);
>> +}
>> +
>> +/* Implement the memtag_to_string gdbarch method. */
>> +
>> +static std::string
>> +aarch64_linux_memtag_to_string (struct gdbarch *gdbarch,
>> + struct value *address,
>> + enum memtag_type tag_type)
>> +{
>> + gdb_assert (address != nullptr);
>> +
>> + struct value *v_tag = aarch64_linux_get_memtag (gdbarch, address, tag_type);
>> +
>> + if (v_tag == nullptr && tag_allocation)
>> + error (_("Error getting tag from target"));
>> +
>> + CORE_ADDR tag = value_as_address (v_tag);
>> +
>> + return string_printf ("0x%s", phex_nz (tag, sizeof (tag)));
>> +}
>
> I'll see in the following patches how gdbarch_linux_memtag_to_string is
> used, but at first glance I think it would make sense for this method to
> have the prototype:
>
> std::string gdbarch_memtag_to_string (struct gdbarch *gdbarch, value *tag);
>
> Meaning that you would pass it a tag you already fetched (using
> gdbarch_get_memtag). This way if you already have a tag as a local
> variable and you want to format it as a string, it doesn't require going
> to the target. I don't think you would need the tag type parameter in
> this case, as we wouldn't care where the tag comes from, we'd just use
> the passed-in value.
>
> Also something that can be done as a follow-up patch.
>

I think it would work. The code has changed a bit since it was first put
together. The intention was to have a gdbarch hook to return a string
representation of a tag so generic GDB code wouldn't have to worry about
touching/manipulating arch-specific data.

Then again, value struct types are somewhat generic already.

>> +
>> static void
>> aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
>> {
>> @@ -1570,6 +1755,31 @@ aarch64_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
>> data associated with the address. */
>> set_gdbarch_significant_addr_bit (gdbarch, 56);
>>
>> + /* MTE-specific settings and hooks. */
>> + if (tdep->has_mte ())
>> + {
>> + /* Register a hook for checking if an address is tagged or not. */
>> + set_gdbarch_tagged_address_p (gdbarch, aarch64_linux_tagged_address_p);
>> +
>> + /* Register a hook for checking if there is a memory tag match. */
>> + set_gdbarch_memtag_matches_p (gdbarch,
>> + aarch64_linux_memtag_matches_p);
>> +
>> + /* Register a hook for setting the logical/allocation tags for
>> + a range of addresses. */
>> + set_gdbarch_set_memtags (gdbarch, aarch64_linux_set_memtags);
>> +
>> + /* Register a hook for extracting the logical/allocation tag from an
>> + address. */
>> + set_gdbarch_get_memtag (gdbarch, aarch64_linux_get_memtag);
>> +
>> + /* Set the allocation tag granule size to 16 bytes. */
>> + set_gdbarch_memtag_granule_size (gdbarch, AARCH64_MTE_GRANULE_SIZE);
>> +
>> + /* Register a hook for converting a memory tag to a string. */
>> + set_gdbarch_memtag_to_string (gdbarch, aarch64_linux_memtag_to_string);
>> + }
>> +
>> /* Initialize the aarch64_linux_record_tdep. */
>> /* These values are the size of the type that will be used in a system
>> call. They are obtained from Linux Kernel source. */
>> diff --git a/gdb/arch/aarch64-mte-linux.c b/gdb/arch/aarch64-mte-linux.c
>> index ede5f5f2b9..4461864a32 100644
>> --- a/gdb/arch/aarch64-mte-linux.c
>> +++ b/gdb/arch/aarch64-mte-linux.c
>> @@ -22,7 +22,8 @@
>> /* See arch/aarch64-mte-linux.h */
>>
>> size_t
>> -aarch64_mte_get_tag_granules (CORE_ADDR addr, size_t len, size_t granule_size)
>> +aarch64_mte_get_tag_granules (CORE_ADDR addr, size_t len,
>> + size_t granule_size)
>
> Unintended change.
>

Oops. Fixed.

>> {
>> /* Start address */
>> CORE_ADDR s_addr = align_down (addr, granule_size);
>> @@ -32,3 +33,41 @@ aarch64_mte_get_tag_granules (CORE_ADDR addr, size_t len, size_t granule_size)
>> /* We always have at least 1 granule. */
>> return 1 + (e_addr - s_addr) / granule_size;
>> }
>> +
>> +/* See arch/aarch64-mte-linux.h */
>> +
>> +CORE_ADDR
>> +aarch64_mte_make_ltag_bits (CORE_ADDR value)
>> +{
>> + return value & AARCH64_MTE_LOGICAL_MAX_VALUE;
>> +}
>> +
>> +/* See arch/aarch64-mte-linux.h */
>> +
>> +CORE_ADDR
>> +aarch64_mte_make_ltag (CORE_ADDR value)
>> +{
>> + return aarch64_mte_make_ltag_bits (value)
>> + << AARCH64_MTE_LOGICAL_TAG_START_BIT;
>
> We would usually use parenthesis when wrapping this:
>
> return (aarch64_mte_make_ltag_bits (value)
> << AARCH64_MTE_LOGICAL_TAG_START_BIT);
>
> The argument being that Emacs aligns it correctly if you do that (not
> that I care :)).

Fair enough. Fixed now.

>
> The patch LGTM with the nits fixed
Thanks!