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From: "i at maskray dot me" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/31076] Extra struct vm_area_struct with ---p created when PAGE_SIZE < max-page-size
Date: Fri, 08 Dec 2023 03:22:29 +0000 [thread overview]
Message-ID: <bug-31076-131-SXDQvQrcJa@http.sourceware.org/bugzilla/> (raw)
In-Reply-To: <bug-31076-131@http.sourceware.org/bugzilla/>
https://sourceware.org/bugzilla/show_bug.cgi?id=31076
--- Comment #21 from Fangrui Song <i at maskray dot me> ---
(In reply to Florian Weimer from comment #20)
> (In reply to Fangrui Song from comment #19)
> > (In reply to Florian Weimer from comment #18)
> > > (In reply to Kalesh Singh from comment #15)
> > > > AIUI if the runtime-page-size equals the max-page-size, the holes are also
> > > > mapped in as part of the segment mapping and share the same permissions.
> > > > Does this mean that on such systems, any protection it offers becomes void?
> > >
> > > That's my understanding. The current behavior gives programmers the *option*
> > > to avoid mapping extra code on small-page systems, while maintaining
> > > compatibility with large-page systems. The proposed change to over-map to
> > > the next load-segment, to fill the gap, would take away that option.
> >
> > Can you elaborate what the option (and "functionality" below) is? Since
> > 22930c9bf21ea15d0da1477a379029e2de259b69 (1996) rtld just calls `mprotect`
> > (one
> > extra `struct vm_area_struct` instance, see `sudo grep vm_area_struct
> > /proc/slabinfo`), and the user has no option to control this behavior.
>
> The developer can produce a binary with a LOAD segment that does not need
> that mprotect call.
This is described by "## Solution 2 - Modify the static linker" in comment 1. I
find the approach intrusive.
("Moreover, a PT_LOAD whose end isn't backed by a section is unusual." in
comment 17.)
A loader alternative is "3. Extend the map end to cover the gap" in comment 17.
Instead of
mmap(l->l_addr+c->mapstart, c->mapend - c->mapstart, c->prot,
MAP_FIXED|MAP_COPY|MAP_FILE, fd, c->mapoff)
increase the map length
mmap(l->l_addr+c->mapstart, c[1].mapstart - c->mapstart, c->prot,
MAP_FIXED|MAP_COPY|MAP_FILE, fd, c->mapoff)
The existing mprotect in glibc/elf/dl-map-segments.h will not be touched.
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next prev parent reply other threads:[~2023-12-08 3:22 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-18 18:48 [Bug dynamic-link/31076] New: " jyescas at google dot com
2023-11-18 18:50 ` [Bug dynamic-link/31076] " jyescas at google dot com
2023-11-21 13:42 ` carlos at redhat dot com
2023-11-22 0:39 ` i at maskray dot me
2023-11-22 4:33 ` kaleshsingh at google dot com
2023-11-22 18:19 ` jyescas at google dot com
2023-11-23 11:42 ` sam at gentoo dot org
2023-11-24 17:40 ` adhemerval.zanella at linaro dot org
2023-11-27 15:11 ` fweimer at redhat dot com
2023-11-27 15:22 ` fweimer at redhat dot com
2023-11-27 16:27 ` adhemerval.zanella at linaro dot org
2023-11-27 17:19 ` fweimer at redhat dot com
2023-11-27 17:39 ` adhemerval.zanella at linaro dot org
2023-11-27 17:45 ` fweimer at redhat dot com
2023-11-27 17:58 ` adhemerval.zanella at linaro dot org
2023-11-27 19:47 ` jyescas at google dot com
2023-11-27 19:55 ` jyescas at google dot com
2023-11-28 8:48 ` rprichard at google dot com
2023-11-28 18:59 ` kaleshsingh at google dot com
2023-11-28 23:58 ` jyescas at google dot com
2023-12-02 17:08 ` i at maskray dot me
2023-12-06 11:57 ` fweimer at redhat dot com
2023-12-07 5:11 ` i at maskray dot me
2023-12-07 9:30 ` fweimer at redhat dot com
2023-12-08 3:22 ` i at maskray dot me [this message]
2024-06-23 18:24 ` corsix at corsix dot org
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