[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 2/7] x86/wait: prevent duplicated assembly labels
On Fri, Mar 14, 2025 at 10:13:07AM +0100, Jan Beulich wrote: > On 14.03.2025 10:05, Andrew Cooper wrote: > > On 14/03/2025 8:44 am, Jan Beulich wrote: > >> On 14.03.2025 09:30, Roger Pau Monné wrote: > >>> On Fri, Mar 14, 2025 at 09:24:09AM +0100, Jan Beulich wrote: > >>>> On 13.03.2025 16:30, Roger Pau Monne wrote: > >>>>> When enabling UBSAN with clang, the following error is triggered during > >>>>> the > >>>>> build: > >>>>> > >>>>> common/wait.c:154:9: error: symbol '.L_wq_resume' is already defined > >>>>> 154 | "push %%rbx; push %%rbp; push %%r12;" > >>>>> | ^ > >>>>> <inline asm>:1:121: note: instantiated into assembly here > >>>>> 1 | push %rbx; push %rbp; push %r12;push %r13; push %r14; > >>>>> push %r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov > >>>>> %rsp,%rsi;.L_wq_resume: rep movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop > >>>>> %r14; pop %r13;pop %r12; pop %rbp; pop %rbx > >>>>> | > >>>>> ^ > >>>>> common/wait.c:154:9: error: symbol '.L_skip' is already defined > >>>>> 154 | "push %%rbx; push %%rbp; push %%r12;" > >>>>> | ^ > >>>>> <inline asm>:1:159: note: instantiated into assembly here > >>>>> 1 | push %rbx; push %rbp; push %r12;push %r13; push %r14; > >>>>> push %r15;sub %esp,%ecx;cmp $4096, %ecx;ja .L_skip;mov > >>>>> %rsp,%rsi;.L_wq_resume: rep movsb;mov %rsp,%rsi;.L_skip:pop %r15; pop > >>>>> %r14; pop %r13;pop %r12; pop %rbp; pop %rbx > >>>>> | > >>>>> > >>>>> ^ > >>>>> 2 errors generated. > >>>>> > >>>>> The inline assembly block in __prepare_to_wait() is duplicated, thus > >>>>> leading to multiple definitions of the otherwise unique labels inside > >>>>> the > >>>>> assembly block. GCC extended-asm documentation notes the possibility of > >>>>> duplicating asm blocks: > >>>>> > >>>>>> Under certain circumstances, GCC may duplicate (or remove duplicates > >>>>>> of) > >>>>>> your assembly code when optimizing. This can lead to unexpected > >>>>>> duplicate > >>>>>> symbol errors during compilation if your asm code defines symbols or > >>>>>> labels. Using ‘%=’ (see AssemblerTemplate) may help resolve this > >>>>>> problem. > >>>>> Move the assembly blocks that deal with saving and restoring the current > >>>>> CPU context into it's own explicitly non-inline functions. This > >>>>> prevents > >>>>> clang from duplicating the assembly blocks. Just using noinline > >>>>> attribute > >>>>> seems to be enough to prevent assembly duplication, in the future > >>>>> noclone > >>>>> might also be required if asm block duplication issues arise again. > >>>> Wouldn't it be a far easier / less intrusive change to simply append %= > >>>> to > >>>> the label names? > >>> That won't work AFAICT, as the inline asm in check_wakeup_from_wait() > >>> won't be able to make a jump to the .L_wq_resume label defined in the > >>> __prepare_to_wait() assembly block if the label is declared as > >>> .L_wq_resume%=. > >>> > >>> Also we want to make sure there's a single .L_wq_resume seeing how > >>> check_wakeup_from_wait() uses it as the restore entry point? > >> Hmm, yes on both points; the %= would only work for .Lskip. Have you gained > >> understanding why there is this duplication? The breaking out of the asm() > >> that you do isn't going to be reliable, as in principle the compiler is > >> still permitted to duplicate stuff. Afaict the only reliable way is to move > >> the code to a separate assembly file (with the asm() merely JMPing there, > >> providing a pseudo-return-address by some custom means). Or to a file-scope > >> asm(), as those can't be duplicated. > > > > See the simplified example in > > https://github.com/llvm/llvm-project/issues/92161 > > > > When I debugged this a while back, The multiple uses of wqv->esp (one > > explicit after the asm, one as an asm parameter) gain pointer > > sanitisation, so the structure looks like: > > > > ... > > if ( bad pointer ) > > __ubsan_report(); > > asm volatile (...); > > if ( bad pointer ) > > __ubsan_report(); > > ... > > > > which then got transformed to: > > > > if ( bad pointer ) > > { > > __ubsan_report(); > > asm volatile (...); > > __ubsan_report(); > > } > > else > > asm volatile (...); > > But isn't it then going to be enough to latch &wqv->esp into a local variable, > and use that in the asm() and in the subsequent if()? I have the following diff which seems to prevent the duplication, would you both be OK with this approach? Thanks, Roger. --- diff --git a/xen/common/wait.c b/xen/common/wait.c index cb6f5ff3c20a..60ebd58a0abd 100644 --- a/xen/common/wait.c +++ b/xen/common/wait.c @@ -124,6 +124,7 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv) struct cpu_info *cpu_info = get_cpu_info(); struct vcpu *curr = current; unsigned long dummy; + void *esp = NULL; ASSERT(wqv->esp == NULL); @@ -166,12 +167,12 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv) ".L_skip:" "pop %%r15; pop %%r14; pop %%r13;" "pop %%r12; pop %%rbp; pop %%rbx" - : "=&S" (wqv->esp), "=&c" (dummy), "=&D" (dummy) + : "=&S" (esp), "=&c" (dummy), "=&D" (dummy) : "0" (0), "1" (cpu_info), "2" (wqv->stack), [sz] "i" (PAGE_SIZE) : "memory", "rax", "rdx", "r8", "r9", "r10", "r11" ); - if ( unlikely(wqv->esp == NULL) ) + if ( unlikely(esp == NULL) ) { gdprintk(XENLOG_ERR, "Stack too large in %s\n", __func__); domain_crash(curr->domain); @@ -179,6 +180,7 @@ static void __prepare_to_wait(struct waitqueue_vcpu *wqv) for ( ; ; ) do_softirq(); } + wqv->esp = esp; } static void __finish_wait(struct waitqueue_vcpu *wqv)
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