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Date: Wed, 26 Feb 2020 21:00:51 -0800
From: Fangrui Song <i@...kray.me>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] Add REL_COPY size change detection

On 2020-02-26, Rich Felker wrote:
>On Wed, Feb 26, 2020 at 08:53:03PM +0100, Florian Weimer wrote:
>> * Rich Felker:
>>
>> > On Wed, Feb 26, 2020 at 07:38:31PM +0100, Florian Weimer wrote:
>> >> * Rich Felker:
>> >>
>> >> > At the very least I think we ought to catch and error on the case
>> >> > where def.sym->st_size>sym->st_size, since we can't honor it and
>> >> > failure to honor it can produce silent memory corruption. I'm less
>> >> > sure about what to do if def.sym->st_size<sym->st-size; this case
>> >> > seems safe and might be desirable not to break (I vaguely recall an
>> >> > intent that it be ok), but if you think there are reasons it's
>> >> > dangerous I'm ok with disallowing it too. I'm having a hard time now
>> >> > thinking of a reason it would really help to support that, anyway.
>> >>
>> >> Unfortunately the Mozilla NSS people disagree that size mismatches for
>> >> global symbols are an ABI break.  I don't know if this is relevant in
>> >> the musl context, but it means that for glibc, we probably can't make
>> >> it a hard error.
>> >>
>> >> I want to have better diagnostics for this in glibc, but the current
>> >> warning (which is poorly worded at that) is in the
>> >> architecture-specific code, and I got side-tracked when I tried to
>> >> clean this up the last time.
>> >
>> > Thanks for the feedback. Do you have a source where we could read more
>> > about this? What non-broken behavior do they expect to get when sizes
>> > don't match?

+1 for a `def.sym->st_size!=sym->st-size` diagnostic.

>> There's an NSS bug report:
>>
>>   <https://bugzilla.mozilla.org/show_bug.cgi?id=1201900>
>>
>> It seems that the NSS situation is better than what I remembered.
>
>Good to know.

There may be instances where the code takes the address of a global variable
but does not actually care about the contents (st_size does not matter).

>
>> > As an aside, I think we should be encouraging distros that are using
>> > PIE to get rid of copy relocations by passing whatever options are
>> > needed (or building gcc with whatever options are needed) to avoid
>> > emitting them in PIE. IIRC I looked this up once but I can't remember
>> > what I found.
>>
>> If I recall correctly, the optimization was a factor when rolling out
>> PIE-by-default in Fedora.  I do not know if we can revert it without
>> switching back to fixed-address builds.
>
>I think this is almost surely premature optimization. In almost all
>cases, if there's software where the performance impact makes a
>difference it can be avoided by giving the affected global data
>objects visibility of hidden (if it's not used outside the main
>program anyway) or protected (if it needs to be externally visible).
>But on x86_64 and aarch64, and to some extent on 32-bit arm as well,
>the performance difference of accessing globals via the got vs
>pc-relative is negligible.

clang has an option -mpie-copy-relocations (the name could be improved),
which enables direct access (usually PC-relative) for -fPIE.

// -target aarch64 -fPIE -O3
        adrp    x8, :got:var
        ldr     x8, [x8, :got_lo12:var]
        ldr     w0, [x8]
        ret
// -target aarch64 -fPIE -O3 -mpie-copy-relocations
        adrp    x8, var
        ldr     w0, [x8, :lo12:var]
        ret

On x86, with R_X86_64_[REX_]GOTPCRELX, the option is still beneficial.

// -O3 -fPIE a.c -Wa,--mrelax-relocations=yes
    0:   48 8b 05 00 00 00 00    mov    0x0(%rip),%rax        # 7 <foo+0x7>
                         3: R_X86_64_REX_GOTPCRELX       var-0x4
    7:   8b 00                   mov    (%rax),%eax
    9:   c3                      retq
// -O3 -fPIE a.c -mpie-copy-relocations
    0:   8b 05 00 00 00 00       mov    0x0(%rip),%eax        # 6 <foo+0x6>
                         2: R_X86_64_PC32        var-0x4
    6:   c3                      retq


Users of -mpie-copy-relocations may compile their applications in a
mostly statically linking mode, with only a few definitions in DSOs. If some
variables are unfortunately really external, R_*_COPY will be produced
by the linker.

>> Even if we did that, the ABI incompatibility will still be there.
>
>Yes. But fixing it would avoid any bugs from fallout of the full
>object not being copyable at runtime in any newly build programs.
>
>> There is also a similar truncation issue for TLS variables, I think.
>
>TLS variables never use copy relocations, except for a short period of
>time on riscv64, where thankfully it was realized to be a mistake and
>reverted. So I don't think this issue applies to TLS.
>
>Rich

https://sourceware.org/bugzilla/show_bug.cgi?id=23825
(GCC 10 riscv should have fixed this.)
(glibc/sysdeps/riscv/dl-machine.h has a hack supporting R_RISCV_COPY on
STT_TLS. No other known ld.so supports it.)

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