Re: [LTP] [PATCH v2 4/8] Hugetlb: Migrating libhugetlbfs icache-hygiene

Petr Vorel
2023-01-25T01:11:27Z

Hi Tarun, all,

> Migrating the libhugetlbfs/testcases/icache-hygiene.c test

> Test Description: Older ppc64 kernels don't properly flush dcache to
> icache before giving a cleared page to userspace. With some exceedingly
> hairy code, this attempts to test for this bug.

> This test will never trigger (obviously) on machines with coherent
> icache and dcache (including x86 and POWER5). On any given run,
> even on a buggy kernel there's a chance the bug won't trigger -
> either because we don't get the same physical page back when we
> remap, or because the icache happens to get flushed in the interim.

> Signed-off-by: Tarun Sahu <tsahu@linux.ibm.com>
...
> +++ b/testcases/kernel/mem/hugetlb/hugemmap/hugemmap15.c
...
> +static void sig_handler(int signum, siginfo_t *si, void *uc)
> +{
> +#if defined(__powerpc__) || defined(__powerpc64__) || defined(__ia64__) || \
> + defined(__s390__) || defined(__s390x__) || defined(__sparc__) || \
> + defined(__aarch64__) || (defined(__riscv) && __riscv_xlen == 64)
NOTE: do we consider riscv32 and mipsel32 as dead?

> + /* On powerpc, ia64, s390 and Aarch64, 0 bytes are an illegal
> + * instruction, so, if the icache is cleared properly, we SIGILL
> + * as soon as we jump into the cleared page
> + */
> + if (signum == SIGILL) {
> + tst_res(TINFO, "SIGILL at %p (sig_expected=%p)", si->si_addr,
> + sig_expected);
> + if (si->si_addr == sig_expected)
> + siglongjmp(sig_escape, SUCC_JMP);
> + siglongjmp(sig_escape, FAIL_JMP + SIGILL);
> + }
> +#elif defined(__i386__) || defined(__x86_64__) || defined(__arm__)
> + /* On x86, zero bytes form a valid instruction:
> + * add %al,(%eax) (i386)
> + * or add %al,(%rax) (x86_64)
> + *
> + * So, behaviour depends on the contents of [ER]AX, which in
> + * turn depends on the details of code generation. If [ER]AX
> + * contains a valid pointer, we will execute the instruction
> + * repeatedly until we run off that hugepage and get a SIGBUS
> + * on the second, truncated page. If [ER]AX does not contain
> + * a valid pointer, we will SEGV on the first instruction in
> + * the cleared page. We check for both possibilities
> + * below.
> + *
> + * On 32 bit ARM, zero bytes are interpreted as follows:
> + * andeq r0, r0, r0 (ARM state, 4 bytes)
> + * movs r0, r0 (Thumb state, 2 bytes)
> + *
> + * So, we only expect to run off the end of the huge page and
> + * generate a SIGBUS.
> + */
> + if (signum == SIGBUS) {
> + tst_res(TINFO, "SIGBUS at %p (sig_expected=%p)", si->si_addr,
> + sig_expected);
> + if (sig_expected
> + && (PALIGN(sig_expected, hpage_size)
> + == si->si_addr)) {
> + siglongjmp(sig_escape, SUCC_JMP);
> + }
> + siglongjmp(sig_escape, FAIL_JMP + SIGBUS);
> + }
> +#if defined(__x86_64__) || defined(__i386__)
> + if (signum == SIGSEGV) {
> +#ifdef __x86_64__
> + void *pc = (void *)((ucontext_t *)uc)->uc_mcontext.gregs[REG_RIP];
> +#else
> + void *pc = (void *)((ucontext_t *)uc)->uc_mcontext.gregs[REG_EIP];
> +#endif
> + tst_res(TINFO, "SIGSEGV at %p, PC=%p (sig_expected=%p)",
> + si->si_addr, pc, sig_expected);
> + if (sig_expected == pc)
> + siglongjmp(sig_escape, SUCC_JMP);
> + siglongjmp(sig_escape, FAIL_JMP + SIGSEGV);
> + }
> +#endif
> +#else
> +#error "Need to setup signal conditions for this arch"

Because this, merged as 15625f6185 ("Hugetlb: Migrating libhugetlbfs
icache-hygiene") broke upcoming LTP release for these archs in Buildroot
toolchains. mipsel32 is quite old, but maybe some other distros still play
with riscv32.

IMHO instead of this there should have been using TST_TEST_TCONF() for
unsupported archs, I'll send patch tomorrow.

Kind regards,
Petr

> +#endif
> +}

...