[Bug c/29838] New: -fstack-protector shouldn't use TLS in freestanding mode

[Bug c/29838] New: -fstack-protector shouldn't use TLS in freestanding mode

[samuel dot thibault at ens-lyon dot org]

Hi,

On some architectures, when given -fstack-protector (which is the default on
ubuntu), gcc generates a TLS reference for the stack guard. For instance, on
linux x86 with a fairly recent version of glibc:

echo 'void f (void) { volatile char a[8]; a[3]; }' | gcc -S -x c -O2
-fstack-protector - -o -

generates a %gs:0x14 reference.

In freestanding mode, this poses problem because the target (typically an OS
kernel) does not necessarily have TLS.  In such case, gcc should default back
to referencing __stack_chk_guard.

Samuel


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           Summary: -fstack-protector shouldn't use TLS in freestanding mode
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: samuel dot thibault at ens-lyon dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug c/29838] -fstack-protector shouldn't use TLS in freestanding mode

[samuel dot thibault at ens-lyon dot org]

------- Comment #1 from samuel dot thibault at ens-lyon dot org  2006-11-15 01:16 -------
Created an attachment (id=12622)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12622&action=view)
braindead patch

Just a small braindead patch, not tested at all, just adds testing flag_hosted.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-11-15 02:50 -------
Seems to me, you should not be using a target that defines
TARGET_THREAD_SSP_OFFSET for -ffreestanding mode.  Also IIRC the x86_64 Linux
has a different TLS base register which fixes this issue there.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[samuel dot thibault at ens-lyon dot org]

------- Comment #3 from samuel dot thibault at ens-lyon dot org  2006-11-15 09:33 -------
Mmm, if I have to use another target for avoiding my default target's specific
stuff, what is the use of -ffreestanding?

Does that mean that we will have to add a linux-kernel target (as opposed to
linux-user target) and build a cross-compiler before building a linux kernel?
(replace "linux" with whatever kernel you want).

And x86_64 Linux just poses the same problem: it emits %fs:0x28 instead of
%gs:0x14, but it's just the same issue.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[tschwinge at gnu dot org]

------- Comment #4 from tschwinge at gnu dot org  2006-11-15 10:11 -------
Cced to Jakub Jelinek, who originally implemented this functionality.  Could
you please comment on this issue?


-- 

tschwinge at gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[jakub at gcc dot gnu dot org]

------- Comment #5 from jakub at gcc dot gnu dot org  2006-11-15 10:23 -------
Linux kernel has this support planned:
http://lkml.org/lkml/2006/08/16/216
http://lkml.org/lkml/2006/08/16/217
http://lkml.org/lkml/2006/08/16/218
http://lkml.org/lkml/2006/08/16/220
http://lkml.org/lkml/2006/08/16/221
http://lkml.org/lkml/2006/08/16/222
Linux -ffreestanding should stay as is.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[samuel dot thibault at ens-lyon dot org]

------- Comment #6 from samuel dot thibault at ens-lyon dot org  2006-11-15 10:30 -------
So you are saying that gcc now imposes (whatever the kernel) kernel-land and
user-land to use the same TLS scheme, and now requires people to build a
cross-compiler before building a kernel from another kernel's userland?  I
thought -ffreestanding was precisely meant to escape such considerations...


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[jakub at gcc dot gnu dot org]

------- Comment #7 from jakub at gcc dot gnu dot org  2006-11-15 10:37 -------
Using %fs:0x28/%gs:0x28 on x86_64-linux resp. %gs:0x14 on i?86-linux is part
of the ABI.  -ffreestanding is not supposed to change the ABI, so if you
don't want to use this ABI, just use a different target (x86_64-elf etc., or
don't use -fstack-protector (nobody forces you to use that).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[jakub at gcc dot gnu dot org]

------- Comment #8 from jakub at gcc dot gnu dot org  2006-11-15 10:41 -------
If you use __thread in -ffreestanding it is the same, you don't get emulated
TLS either.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[samuel dot thibault at ens-lyon dot org]

------- Comment #9 from samuel dot thibault at ens-lyon dot org  2006-11-15 11:01 -------
About not using -fstack-protector, the problem is that it is the default on
ubuntu for instance.  That would mean we have to explicitely use
-fno-stack-protector, but only for recent versions of gcc, so we'll have to
detect that, etc... Not counting all such new options that may arise which we'd
have to disable...

Please answer this, at least by just yes/no: you're saying that -ffreestanding
doesn't mean "an OS kernel" (as manual says), but "the kernel of the target",
so that people working on other kernels will have to first build a
cross-compiler? (the bug is a documentation bug then)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[tschwinge at gnu dot org]

------- Comment #10 from tschwinge at gnu dot org  2006-12-15 19:30 -------
Roland McGrath proposed the following: ``I think it really ought to be
controlled by a -mno-stack-protector-tls or suchlike, for complete flexibility.
 Obviously it should default to disabled for -ffreestanding.''


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

[Bug target/29838] -fstack-protector shouldn't use TLS in freestanding mode

[egall at gwmail dot gwu.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=29838

Eric Gallager <egall at gwmail dot gwu.edu> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |egall at gwmail dot gwu.edu

--- Comment #11 from Eric Gallager <egall at gwmail dot gwu.edu> ---
If changing the code generation or adding a new flag to control this is too
difficult, how about just adding this to the list of things that
"-Wstack-protector" warns about in the meantime?