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5b188cc486
Disable strict aliasing, as has been done in the kernel proper for decades
(literally since before git history) to fix issues where gcc will optimize
away loads in code that looks 100% correct, but is _technically_ undefined
behavior, and thus can be thrown away by the compiler.
E.g. arm64's vPMU counter access test casts a uint64_t (unsigned long)
pointer to a u64 (unsigned long long) pointer when setting PMCR.N via
u64p_replace_bits(), which gcc-13 detects and optimizes away, i.e. ignores
the result and uses the original PMCR.
The issue is most easily observed by making set_pmcr_n() noinline and
wrapping the call with printf(), e.g. sans comments, for this code:
printf("orig = %lx, next = %lx, want = %lu\n", pmcr_orig, pmcr, pmcr_n);
set_pmcr_n(&pmcr, pmcr_n);
printf("orig = %lx, next = %lx, want = %lu\n", pmcr_orig, pmcr, pmcr_n);
gcc-13 generates:
0000000000401c90 <set_pmcr_n>:
401c90: f9400002 ldr x2, [x0]
401c94: b3751022 bfi x2, x1, #11, #5
401c98: f9000002 str x2, [x0]
401c9c: d65f03c0 ret
0000000000402660 <test_create_vpmu_vm_with_pmcr_n>:
402724: aa1403e3 mov x3, x20
402728: aa1503e2 mov x2, x21
40272c: aa1603e0 mov x0, x22
402730: aa1503e1 mov x1, x21
402734: 940060ff bl 41ab30 <_IO_printf>
402738: aa1403e1 mov x1, x20
40273c: 910183e0 add x0, sp, #0x60
402740: 97fffd54 bl 401c90 <set_pmcr_n>
402744: aa1403e3 mov x3, x20
402748: aa1503e2 mov x2, x21
40274c: aa1503e1 mov x1, x21
402750: aa1603e0 mov x0, x22
402754: 940060f7 bl 41ab30 <_IO_printf>
with the value stored in [sp + 0x60] ignored by both printf() above and
in the test proper, resulting in a false failure due to vcpu_set_reg()
simply storing the original value, not the intended value.
$ ./vpmu_counter_access
Random seed: 0x6b8b4567
orig = 3040, next = 3040, want = 0
orig = 3040, next = 3040, want = 0
==== Test Assertion Failure ====
aarch64/vpmu_counter_access.c:505: pmcr_n == get_pmcr_n(pmcr)
pid=71578 tid=71578 errno=9 - Bad file descriptor
1 0x400673: run_access_test at vpmu_counter_access.c:522
2 (inlined by) main at vpmu_counter_access.c:643
3 0x4132d7: __libc_start_call_main at libc-start.o:0
4 0x413653: __libc_start_main at ??:0
5 0x40106f: _start at ??:0
Failed to update PMCR.N to 0 (received: 6)
Somewhat bizarrely, gcc-11 also exhibits the same behavior, but only if
set_pmcr_n() is marked noinline, whereas gcc-13 fails even if set_pmcr_n()
is inlined in its sole caller.
Cc: stable@vger.kernel.org
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116912
Signed-off-by: Sean Christopherson <seanjc@google.com>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.