linux-kernel - Re: [RFC PATCH 1/3] rseq: Introduce feature size and alignment ELF auxiliary vector entries

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Message-ID: <1576236784.46087.1644191636055.JavaMail.zimbra@efficios.com>
Date:   Sun, 6 Feb 2022 18:53:56 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fw@...eb.enyo.de>,
        David Laight <David.Laight@...lab.com>,
        carlos <carlos@...hat.com>, Peter Oskolkov <posk@...k.io>,
        libc-coord@...ts.openwall.com
Subject: Re: [RFC PATCH 1/3] rseq: Introduce feature size and alignment ELF
 auxiliary vector entries

----- On Feb 6, 2022, at 4:49 PM, Peter Zijlstra peterz@...radead.org wrote:

> On Thu, Feb 03, 2022 at 02:38:51PM -0500, Mathieu Desnoyers wrote:
> 
>> @@ -286,6 +287,10 @@ create_elf_tables(struct linux_binprm *bprm, const struct
>> elfhdr *exec,
>>  	if (bprm->have_execfd) {
>>  		NEW_AUX_ENT(AT_EXECFD, bprm->execfd);
>>  	}
>> +#ifdef CONFIG_RSEQ
>> +	NEW_AUX_ENT(AT_RSEQ_FEATURE_SIZE, offsetof(struct rseq, end));
> 
> I've gotta ask, what's up with offsetof(, end) vs sizeof() ?

sizeof() includes the 12 bytes of padding at the end of struct rseq, for a
total of 32 bytes (currently). offsetof(, end) is currently 20 bytes, which
is the offset exactly after the last field.

For the "feature size" (meaning the populated fields), we really want a size
that excludes padding.

Thanks,

Mathieu

> 
>> +	NEW_AUX_ENT(AT_RSEQ_ALIGN, __alignof__(struct rseq));
> > +#endif

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com