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Re: [PATCH 0/3] linux-user: Implement x86_64 vsyscalls

From: Alex Bennée
Subject: Re: [PATCH 0/3] linux-user: Implement x86_64 vsyscalls
Date: Thu, 16 Jan 2020 14:05:27 +0000
User-agent: mu4e 1.3.6; emacs 28.0.50

Richard Henderson <address@hidden> writes:

> The x86_64 abi has a legacy vsyscall page.  The kernel folk
> have been trying to deprecate this since at least v3.1, but
> (1) We don't implement the vdso that replaces vsyscalls,
> (2) As of v5.5, the vsyscall page is still enabled by default.
> This lack is affecting Peter's linux-user testing.
> The dependency is not obvious because Peter is running the tests
> on x86_64, so the host is providing a vsyscall page to qemu.
> Because of how user-only memory operations are handled, with no
> validation of guest vs host pages, so long as qemu chooses to
> run with guest_base == 0, the guest may Just So Happen to read
> the host's vsyscall page.
> Complicating this, new OS releases may use a kernel configured
> with CONFIG_LEGACY_VSYSCALL_XONLY=y, which means the the vsyscall
> page cannot be read, only executed.  Which means that the guest
> then cannot read the host vsyscall page during translation and
> will SIGSEGV.
> Exactly which of these many variables is affecting Peter's testing
> with Ubuntu 18.04 of my TCG merge, I'm not exactly sure.  I suspect
> that it is the change to drop the textseg_addr adjustment to user-only
> static binaries.  IIRC bionic does not support -static-pie, which is
> the preferred replacement.  This could mean that the host and guest
> binaries overlap, which leads to guest_base != 0.
> I vaguely remember someone (Paolo?) implementing something like
> this many years ago, but clearly it never got merged.
> In any case, this emulation has been missing for too long.

It turns out the /sbin/ldconfig crash is a regression w.r.t from
stretch->buster so unrelated to these patches. However I've been giving
them a spin with the linux vdso selftests and stuff is breaking which I
guess means it's incomplete?

alex@23eb55f27ff8:~/lsrc/linux.git/tools/testing/selftests/x86$ make test_vdso
gcc -m64 -o /home/alex/lsrc/linux.git/tools/testing/selftests/x86/test_vdso_64 
-O2 -g -std=gnu99 -pthread -Wall -no-pie -DCAN_BUILD_64 test_vdso.c -lrt -ldl
alex@23eb55f27ff8:~/lsrc/linux.git/tools/testing/selftests/x86$ ./test_vdso_64 
[WARN]  failed to find vDSO
[RUN]   Testing clock_gettime for clock CLOCK_REALTIME (0)...
qemu: uncaught target signal 11 (Segmentation fault) - core dumped
Segmentation fault (core dumped)

Also from selftests/vDSO:

alex@23eb55f27ff8:~/lsrc/linux.git/tools/testing/selftests/vDSO$ ./vdso_test 
AT_SYSINFO_EHDR is not present!

vdso_standalone_test_x86 just exits with a non-zero value. I'm still
looking into that one.

> r~
> Richard Henderson (3):
>   target/i386: Renumber EXCP_SYSCALL
>   linux-user/i386: Split out gen_signal
>   linux-user/i386: Emulate x86_64 vsyscalls
>  target/i386/cpu.h          |   6 +-
>  linux-user/i386/cpu_loop.c | 197 ++++++++++++++++++++++++++-----------
>  target/i386/translate.c    |  16 ++-
>  3 files changed, 155 insertions(+), 64 deletions(-)

Alex Bennée

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