Discussion:
Some questions about compiler work on x86_64-win64
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Bishop via fpc-devel
2017-03-16 22:06:55 UTC
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Sven Barth via fpc-devel
2017-03-17 09:56:47 UTC
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Am 17.03.2017 08:08 schrieb "Bishop via fpc-devel" <
I have some questions about compiler work on x86_64-win64.
1) EXE-file contained .CRT section. As i understand it contained one
pointer to _FPC_Tls_Callback functions from RTL. Is it used only for this?

Correct.
2) Why INITFINAL table on both Windows and Linux puted to .data section?
Is we need to change it in time of app run? Or even possible it must be
evicted and replace just with call instructions? I mean autogeneration of
fpc_InitializeUnits and fpc_FinalizeUnits?

It probably should be .rdata, but I think that older binutils had problems
with that, thus they were put into .data and that was never changed till
now...
3) Why .fpc.n_version puted to .data section? Is we need change it on
runtime? Possible must be in .rdata? On linux it is entyre section for it,
does this is necessary?

Same as above.
4) Why referencing to global variable under Windows all time compiled
like RIP-related, even if no Generate-PIC enable (on Linux work fine)?

I might remember it incorrectly, but I think that Microsoft says that all
accesses should be RIP relative to reduce the need for relocations.

Regards,
Sven
Jonas Maebe
2017-03-17 16:25:42 UTC
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Post by Sven Barth via fpc-devel
I might remember it incorrectly, but I think that Microsoft says that
all accesses should be RIP relative to reduce the need for relocations.
Probably also for ASLR.


Jonas

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Bishop
2017-03-17 21:16:40 UTC
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Sven Barth via fpc-devel
2017-03-17 22:51:05 UTC
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03/17/17 11:56:47, Sven Barth via fpc-devel <
Post by Sven Barth via fpc-devel
1) EXE-file contained .CRT section. As i understand it contained one
pointer to _FPC_Tls_Callback functions from RTL. Is it used only for this?
Post by Sven Barth via fpc-devel
Correct.
And main sence of this function is allocated ThreadVar regions for
thread, than was created in initialization code of DLL`s that was loaded
staticaly (OS load them use information in .idata section)?

Bo, the main sense of this is to detect when a new thread is started and
more importantly terminated cause only with this we can free the threadvar
area of the thread accordingly (if the thread is an external one, not one
started using BeginThread or TThread).
Post by Sven Barth via fpc-devel
It probably should be .rdata, but I think that older binutils had
problems with that, thus they were put into .data and that was never
changed till now...
So now it is possible to change it? Plus what about 2nd part of my
question?
Post by Sven Barth via fpc-devel
Or even possible it must be evicted and replace just with call
instructions? I mean autogeneration of fpc_InitializeUnits and
fpc_FinalizeUnits?
What reason for we need so strange init/final unit function calling
solution? Autogenerated functions will take less space (direct or
RIP-related address - all functions in same segment), safer than varible in
w/r segment (this is a way to make some atack on application) and faster.

Why *should* it be auto generated if one can use a table and let the RTL do
the rest.
Also with the addition of dynamic packages this will move even more towards
a table based approach.
Post by Sven Barth via fpc-devel
I might remember it incorrectly, but I think that Microsoft says that
all accesses should be RIP relative to reduce the need for relocations.
But EXE generated by FPC dont ASLR aware. This have no needed flags in
EXE file header (I use VMMap to get info about process and it show ASLR
modules). Windows support both type of executable (i dont say about DLL in
this time). Static and ASLR. I think it will be logicaly if FPC can
generate both type of applications (it already have flags for PIC and
RIP-related addressing is a good form of PIC realization). Plus on Linux we
have picture like it. Main binary can be PIC or not-PIC. SO-libraries can
be only PIC (that logicaly because they need relocation, that exe-file can
avoid).

But you can set the corresponding PE flag for ASLR using $SetPEOpts (or
so). No recompilation needed in that case.
Microsoft recommended that approach for Win64 so why should we do the work
and implement it differently even if ASLR isn't enabled by default for FPC
executables?

Regards,
Sven
Bishop
2017-03-18 22:11:19 UTC
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Sven Barth via fpc-devel
2017-03-19 08:38:35 UTC
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03/18/17 00:51:05, Sven Barth via fpc-devel <
Post by Sven Barth via fpc-devel
Bo, the main sense of this is to detect when a new thread is started
and more importantly terminated cause only with this we can free the
threadvar area of the thread accordingly (if the thread is an external one,
not one started using BeginThread or TThread).
Thanks, now i understand how its work. Plus as i understand on Linux (and
other unixes) threadvar for external threads allocated on first access to
them (and free via PThread ability to call destructor for key).

Correct.
(and my first word should have been "No", not "Bo"; stupid smartphone
keyboard)
Post by Sven Barth via fpc-devel
Why *should* it be auto generated if one can use a table and let the
RTL do the rest.
Is it not better make all that can be done in compile time? Its not more
complex solution for compiler code, but as i see it, its more harmonious
(Its depend not only INIFINAL, but all tables, than used in RTL to make
work of compiler/linker. As example, FPC_THREADVARTABLES. Different
modules, i mean DLL or SO, use different TLS keys for their threadvar
regions. But why position of variable from begin of threadvar region must
be generated in runtime? Isn`t it work for linker?). Possible this is
depend on that "dynamic packages"?

If you have different modules (the binary and the libraries) then they are
*separate* entities. Cause it could be that a Pascal library is used with a
C binary and thus a library has the whole RTL statically linked (or at
least that part that is used).
Only dynamic packages allow one to transparently have units be part of
different binary modules yet providing one whole application. Package
libraries can however only be used by a binary compiled with the same
compiler as they rely on quite a bit of compiler magic.
Post by Sven Barth via fpc-devel
Also with the addition of dynamic packages this will move even more
towards a table based approach.
Where i can read information about what is it and why we need it? What
kind of problems is must solve? Because we already have dll/so, and as i
know and see for now its enough. Possible my knowledge is not enough to see
whole problem.

With dynamic packages you can share classes, strings, memory, etc. between
the modules (the main binary and the different package libraries), because
the RTL will only exist once. And all this transparently for the user.
When you use ordinary libraries you need to use a shared memory manager to
pass strings around and you can't use the "as" and "is" operators inside
the main binary on classes passed in from the libraries (and by extension
this also applies to exceptions).
Post by Sven Barth via fpc-devel
But you can set the corresponding PE flag for ASLR using $SetPEOpts (or
so). No recompilation needed in that case.
Can. But what if i dont want ASLR binary? Its totaly valid.
Since ASLR is disabled by default in FPC that question is useless.
Post by Sven Barth via fpc-devel
Microsoft recommended that approach for Win64 so why should we do the
work and implement it differently even if ASLR isn't enabled by default for
FPC executables?
Recommendation in not a law (like it is with SEH in Win64). C compilers
allow both type of programs, depend on what programmer need. Is it need
many work to change it? As i see it, its just one check in compiler code
for global varibles (if select PIC - use RIP-related, if not - use direct).
It already done in linux. I think it was better to give compiler user more
possibilities when its cost almoust nothing.

If it is so important to you: patches are welcome. But keep in mind that
the default needs to be the status quo.

Regards,
Sven

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