linux-kernel - RE: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and thread creation (v10)

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Message-ID: <69a53ec2ce184af29c4cae58e0b2fb57@AcuMS.aculab.com>
Date:   Fri, 14 Jun 2019 13:29:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Mathieu Desnoyers' <mathieu.desnoyers@...icios.com>,
        Florian Weimer <fweimer@...hat.com>
CC:     carlos <carlos@...hat.com>, Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: RE: [PATCH 1/5] glibc: Perform rseq(2) registration at C startup and
 thread creation (v10)

From: Mathieu Desnoyers
> Sent: 14 June 2019 14:02
...
> But my original issue remains: if I define a variable called __rseq_handled
> within either the main executable or the preloaded library, it overshadows
> the libc one:

1) That is the was elf symbol resolution is required to work.
   Otherwise variables like 'errno' (non-thread safe form) wouldn't work.

2) Don't do it then :-)
   Names starting with __ will be reserved (probably 'for the implementation').

The real 'fun' starts because, under some circumstances, looking up a symbol as:
	foo = dlsym(lib_handle, "foo");
Can find the data item instead of the function!
Usually it works (even when foo is global data) because 'lib_handle' refers
to a different symbol table.
But it can go horribly wrong.

	David

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