Messages in this thread Patch in this message | | | Subject | Re: [PATCH 2/2] sigaltstack: remove EPERM check to make swapcontext() usable | From | Stas Sergeev <> | Date | Sun, 31 Jan 2016 05:45:56 +0300 |
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09.01.2016 05:49, Stas Sergeev пишет: > 09.01.2016 05:03, Andy Lutomirski пишет: >> On Fri, Jan 8, 2016 at 5:18 PM, Stas Sergeev <stsp@list.ru> wrote: >>> linux implements the sigaltstack() in a way that makes it impossible to >>> use with swapcontext(). Per the man page, sigaltstack is allowed to return >>> EPERM if the process is altering its sigaltstack while running on >>> sigaltstack. >>> This is likely needed to consistently return oss->ss_flags, that indicates >>> whether the process is being on sigaltstack or not. >>> Unfortunately, linux takes that permission to return EPERM too literally: >>> it returns EPERM even if you don't want to change to another sigaltstack, >>> but only want to disable sigaltstack with SS_DISABLE. >>> You can't use swapcontext() without disabling sigaltstack first, or the >>> stack will be re-used and overwritten by a subsequent signal. >>> >>> With this patch, disabling sigaltstack inside a signal handler became >>> possible, and the swapcontext() can then be used safely. The oss->ss_flags >>> will then return SS_DISABLE, which doesn't seem to contradict the >>> (very ambiguous) man page wording, namely: >>> SS_ONSTACK >>> The process is currently executing on the alternate signal >>> stack. (Note that it is not possible to change the alternate >>> signal stack if the process is currently executing on it.) >> You're definitely contradicting the "Note" part, though. POSIX is >> quite clear, too: >> >> "Attempts to modify the alternate signal stack while the process is >> executing on it fail." > "modify" may not include "disable". > You don't modify the stack's location or size, just temporary disable its use. > So I believe SS_DISABLE should be fine. > Of course this is just one of the possible interpretations. > But it is the one that looks simple and satisfies everyone. I've finally made the patch to demonstrate that. It has the nasty disadvantage of touching the arch-specific code, but it has the advantages too: - seems compatible with both posix and swapcontext() (before using swapcontext() in a sighandler, the sigaltstack have to be disabled - this is what this patch makes possible) - doesn't require the new flag to paper around one of the possible posix interpretation - consistently returns oss->ss_flags==SS_ONSTACK while in sighandler, even after setting flags to SS_DISABLE - allows someone (like glibc), if need be, to implement the old behaviour: it can check for oss->ss_flags first, and if it is SS_ONSTACK, return EPERM without trying to set the new value.
So since this patch demonstrates the way of solving the swapcontext() problem without adding the SS_FORCE flag to sigaltstack(), I believe we shouldn't be adding it. That flag will only allow us to have a smaller patch that doesn't touch the arch code, but is that a good enough justification?
So if there are no objections, I'll probably have to add an arch-specific ifdefs to this patch to make other arches unaffected, and submit it. Of course we can still just remove the EPERM check. Thoughts?
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c index cb6282c..06e2591 100644 --- a/arch/x86/kernel/signal.c +++ b/arch/x86/kernel/signal.c @@ -216,7 +216,7 @@ get_sigframe(struct k_sigaction *ka, struct pt_regs *regs, size_t frame_size, if (!onsigstack) { /* This is the X/Open sanctioned signal stack switching. */ if (ka->sa.sa_flags & SA_ONSTACK) { - if (current->sas_ss_size) + if (sas_ss_enabled()) sp = current->sas_ss_sp + current->sas_ss_size; } else if (config_enabled(CONFIG_X86_32) && (regs->ss & 0xffff) != __USER_DS && diff --git a/include/linux/sched.h b/include/linux/sched.h index edad7a4..f7e7026 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1572,6 +1572,7 @@ struct task_struct {
unsigned long sas_ss_sp; size_t sas_ss_size; + int sas_ss_flags;
struct callback_head *task_works;
@@ -2550,8 +2551,16 @@ static inline int sas_ss_flags(unsigned long sp) { if (!current->sas_ss_size) return SS_DISABLE; + if (on_sig_stack(sp)) + return SS_ONSTACK; + if (current->sas_ss_flags == SS_DISABLE) + return SS_DISABLE; + return 0; +}
- return on_sig_stack(sp) ? SS_ONSTACK : 0; +static inline int sas_ss_enabled(void) +{ + return (current->sas_ss_size && current->sas_ss_flags != SS_DISABLE); }
static inline unsigned long sigsp(unsigned long sp, struct ksignal *ksig) diff --git a/include/linux/signal.h b/include/linux/signal.h index 92557bb..844b113 100644 --- a/include/linux/signal.h +++ b/include/linux/signal.h @@ -432,7 +432,7 @@ int __save_altstack(stack_t __user *, unsigned long); stack_t __user *__uss = uss; \ struct task_struct *t = current; \ put_user_ex((void __user *)t->sas_ss_sp, &__uss->ss_sp); \ - put_user_ex(sas_ss_flags(sp), &__uss->ss_flags); \ + put_user_ex(t->sas_ss_flags, &__uss->ss_flags); \ put_user_ex(t->sas_ss_size, &__uss->ss_size); \ } while (0);
diff --git a/kernel/fork.c b/kernel/fork.c index fce002e..e5edc5c 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1483,8 +1483,10 @@ static struct task_struct *copy_process(unsigned long clone_flags, /* * sigaltstack should be cleared when sharing the same VM */ - if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) + if ((clone_flags & (CLONE_VM|CLONE_VFORK)) == CLONE_VM) { p->sas_ss_sp = p->sas_ss_size = 0; + p->sas_ss_flags = SS_DISABLE; + }
/* * Syscall tracing and stepping should be turned off in the diff --git a/kernel/signal.c b/kernel/signal.c index f3f1f7a..ef537d8 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -3101,6 +3101,7 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s void __user *ss_sp; size_t ss_size; int ss_flags; + int onsigstack;
error = -EFAULT; if (!access_ok(VERIFY_READ, uss, sizeof(*uss))) @@ -3111,32 +3112,48 @@ do_sigaltstack (const stack_t __user *uss, stack_t __user *uoss, unsigned long s if (error) goto out;
- error = -EPERM; - if (on_sig_stack(sp)) - goto out; - error = -EINVAL; - /* - * Note - this code used to test ss_flags incorrectly: - * old code may have been written using ss_flags==0 - * to mean ss_flags==SS_ONSTACK (as this was the only - * way that worked) - this fix preserves that older - * mechanism. - */ if (ss_flags != SS_DISABLE && ss_flags != SS_ONSTACK && ss_flags != 0) goto out;
- if (ss_flags == SS_DISABLE) { - ss_size = 0; - ss_sp = NULL; - } else { + onsigstack = on_sig_stack(sp); + if (ss_size == 0) { + switch (ss_flags) { + case 0: + error = -EPERM; + if (onsigstack) + goto out; + current->sas_ss_sp = 0; + current->sas_ss_size = 0; + current->sas_ss_flags = SS_DISABLE; + break; + case SS_ONSTACK: + /* re-enable previously disabled sas */ + error = -EINVAL; + if (current->sas_ss_size == 0) + goto out; + break; + default: + break; + } + } else if (ss_flags != SS_DISABLE) { + error = -EPERM; + if (onsigstack) + goto out; error = -ENOMEM; if (ss_size < MINSIGSTKSZ) goto out; + current->sas_ss_sp = (unsigned long) ss_sp; + current->sas_ss_size = ss_size; + /* unfortunately POSIX forces us to treat 0 + * as SS_ONSTACK here, and some legacy apps + * perhaps used that... */ + if (ss_flags == 0) + ss_flags = SS_ONSTACK; }
- current->sas_ss_sp = (unsigned long) ss_sp; - current->sas_ss_size = ss_size; + if (ss_flags != 0) + current->sas_ss_flags = ss_flags; }
error = 0; @@ -3168,7 +3185,7 @@ int __save_altstack(stack_t __user *uss, unsigned long sp) { struct task_struct *t = current; return __put_user((void __user *)t->sas_ss_sp, &uss->ss_sp) | - __put_user(sas_ss_flags(sp), &uss->ss_flags) | + __put_user(t->sas_ss_flags, &uss->ss_flags) | __put_user(t->sas_ss_size, &uss->ss_size); }
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