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Message-ID: <20250808024428.GR1827@brightrain.aerifal.cx>
Date: Thu, 7 Aug 2025 22:44:28 -0400
From: Rich Felker <dalias@...c.org>
To: Thorsten Glaser <tg@...lvis.org>
Cc: musl@...ts.openwall.com, David Edelsohn <dje.gcc@...il.com>,
Sertonix <sertonix@...teo.net>
Subject: Re: ctr registry after syscall on powerpc
On Fri, Aug 08, 2025 at 02:32:16AM +0200, Thorsten Glaser wrote:
> On Thu, 7 Aug 2025, Rich Felker wrote:
>
> >> > The relevant code from src/malloc/mallocng/malloc.c:
> >> >
> >> > size_t pagesize = PGSZ; // stored into ctr registry
> >> > ...
> >> > ctx.brk = brk(0); // mess up ctr registry
> >> > ...
> >> > ctx.brk += -ctx.brk & (pagesize-1); // try to read back ctr
> >> > registry
>
> >It's not across a[n exteral] call. It's just across an (inlineable)
> >syscall. So apparently this is a bug in our syscall asm constraints.
>
> Is brk() an inlineable syscall that frees GCC from having to save
> registers it can clobber?
>
> Hm, perhaps if it can “see” the entire implementation, but that may
> be surprising to users. But… I mean, it’s not static inline or a
> macro, is it?
>
> (Just adding that clobber is ofc good.)
In this particular code it is. There is no functional brk() in musl.
It's not a supported programming model for various reasons that mostly
amount to it being catastrophically-breaking if called in a program
that uses malloc, but "uses malloc" is not really a defined concept
since various libc interfaces may use malloc internally.
The brk() used internally in mallocng is a macro defined in the
host-environment-glue include file glue.h, and it expands to
essentially __syscall(SYS_brk,p) where __syscall is a macro expanding
to a call to the __syscall1 static inline function whose body is an
asm statement. For a non-libc-internal use of mallocng you'd generally
define it to just be always-failing unless the system has a brk you
can safely use.
Rich
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