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([2804:7f0:8284:1487:e8:f2cf:c3e1:1857]) by smtp.gmail.com with ESMTPSA id x5sm6663292qkf.44.2020.10.26.07.59.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 Oct 2020 07:59:31 -0700 (PDT) Subject: Re: [PATCH v2 20/24] Documentation for the new mtag commands To: Eli Zaretskii References: <20201022200014.5189-1-luis.machado@linaro.org> <20201022200014.5189-21-luis.machado@linaro.org> <837drhla9o.fsf@gnu.org> <83y2jwj0dk.fsf@gnu.org> Message-ID: <53e13a05-ee8d-063a-fad7-b1fe3b43ad6c@linaro.org> Date: Mon, 26 Oct 2020 11:59:29 -0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , From: Luis Machado via Gdb-patches Reply-To: Luis Machado Cc: david.spickett@linaro.org, gdb-patches@sourceware.org Errors-To: gdb-patches-bounces@sourceware.org Sender: "Gdb-patches" On 10/23/20 4:04 PM, Luis Machado wrote: > On 10/23/20 2:52 PM, Eli Zaretskii wrote: >>> Cc: gdb-patches@sourceware.org, david.spickett@linaro.org >>> From: Luis Machado >>> Date: Fri, 23 Oct 2020 11:33:36 -0300 >>> >>> But, in general, there will always be a memory-side tag against which a >>> logical tag (contained in a pointer, for example) will be matched >>> against. >> >> This is the crucial aspect that should be stated, IMO. >> > > I realized the order of the phrases was a bit off. I reordered it a bit > and added to it. How does the following look? > > "Memory tagging is a memory protection technology that uses a pair of > tags to validate memory accesses through pointers.  The tags are integer > values usually comprised of a few bits, depending on the architecture. > > There are two types of tags that are used in this setup: logical and > allocation.  A logical tag is stored in the pointers themselves, usually > at the  higher bits of the pointers.  An allocation tag is the tag > associated with particular ranges of memory in the physical address > space, against which the logical tags from pointers are compared. > > The pointer tag (logical tag) must match the memory tag (allocation tag) > for the memory access to be valid.  If the logical tag does not match > the allocation tag, that will raise a memory violation. > > Allocation tags cover multiple contiguous bytes of physical memory. This > range of bytes is called a memory tag granule and is > architecture-specific.  For example,  AArch64 has a tag granule of 16 > bytes, meaning each allocation tag spans 16 bytes of memory. > > If the underlying architecture supports memory tagging, like AArch64 MTE > or SPARC ADI do,  @value{GDBN} can make use of it to validate addresses > and pointers against memory allocation tags. > > A command prefix of @code{mtag} gives access to the various memory > tagging commands." > > >>>>> +@kindex mtag setltag >>>>> +@item mtag setltag @var{address_expression} @var{tag_bytes} >>>>> +Print the address given by @var{address_expression}, augmented >>>>> with a logical >>>> >>>> It is strange for a command whose name is "set..." to print >>>> something.  I'd expect it to set something instead.  is the above >>>> description correct? >>>> >>> >>> Yes. This is one area that I'd welcome some discussion/feedback. >>> >>> We don't always have a modifiable value as an argument to the "mtag >>> setltag" command. We could have a constant value, a read-only value, >>> some reference or some expression containing multiple pointers. >>> >>> Plus, the most natural way to modify a value in GDB is through the >>> existing "set variable" command. >>> >>> The main goal is to be able to augment a particular address with a given >>> logical tag. That augmented value can then be used to set a particular >>> pointer or value. It will be stored in the history anyway, so that's >>> already a value that you can use. >>> >>> There won't be much reason to set logical tags other than if you're >>> chasing bugs and trying to cause one. It is one additional knob so that >>> you won't need to craft the tagged pointer by hand. >> >> Maybe the command should be called something other than "set...", >> then? >> > > Maybe. Though honestly I'm not really sure what to call it. Even if we > call it "set" and make it really set some variable's value, it won't be > able to set the value of an expression with multiple variables, for > example. > > I'll have to think about it. > David Spicket (cc-ed, handling LLDB's MTE enablement), has suggested "mtag withltag" as opposed to "mtag setltag", which implies we will only display the modified/tagged version of a particular address expression, without setting any value. How does that sound?