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On Wed, May 11, 2011 at 02:31:23PM -0400, Ryan Johnson wrote:By fork semantics the condition *is* a failure. If we try to relax the requirement we risk Bad Things happening, usually in hard-to-diagnose ways. The example I have right off is libgcc_s storing pointers to other dlls and seg faulting when it tries to access pointers which were valid in the parent but not in the child. I prefer a fail-fast solution over cross-fingers-and-hope-it-doesn't-happen-to-me.Hi all,
This is the first of a series of patches, sent in separate emails as requested.
The first patch allows a child which failed due to address space clobbers to report cleanly back to the parent. As a result, DLL_LINK which land wrong, DLL_LOAD whose space gets clobbered, and failure to replicate the cygheap, generate retries and dispense with the terminal spam. Handling of unexpected errors should not have changed. Further, the patch fixes several sources of access violations and crashes, including: - accessing invalid state after failing to notice that a statically-linked dll loaded at the wrong location - accessing invalid state while running dtors on a failed forkee. I follow cgf's approach of simply not running any dtors, based on the observation that dlls in the parent (gcc_s!) can store state about other dlls and crash trying to access that state in the child, even if they appeared to map properly in both processes. - attempting to generate a stack trace when somebody in the call chain used alloca(). This one is only sidestepped here, because we eliminate the access violations and api_fatal calls which would have triggered the problematic stack traces. I have a separate patch which allows offending functions to disable stack traces, if folks are interested, but it was kind of noisy so I left it out for now (cygwin uses alloca pretty liberally!).
Ryan diff --git a/child_info.h b/child_info.h --- a/child_info.h +++ b/child_info.h @@ -92,6 +92,18 @@ public: void alloc_stack_hard_way (volatile char *); };
+/* Several well-known problems can prevent us from patching up a + forkee; when such errors arise the child should exit cleanly (with + a failure code for the parent) rather than dumping stack. */ +#define fork_api_fatal(fmt, args...) \ + do \ + { \ + sigproc_printf (fmt,## args); \ + fork_info->handle_failure (-1); \ + } \ + while(0) + + class fhandler_base;
class cygheap_exec_info diff --git a/dll_init.cc b/dll_init.cc --- a/dll_init.cc +++ b/dll_init.cc @@ -19,6 +19,7 @@ details. */ #include "dtable.h" #include "cygheap.h" #include "pinfo.h" +#include "child_info.h" #include "cygtls.h" #include "exception.h" #include<wchar.h> @@ -131,10 +132,16 @@ dll_list::alloc (HINSTANCE h, per_proces { if (!in_forkee) d->count++; /* Yes. Bump the usage count. */ + else if (d->handle != h) + fork_api_fatal ("Location of %W changed from %p (parent) to %p (child)", + d->name, d->handle, h);You seem to be guranteeing a failure in a condition which could conceivably work ok for simple applications, i.e., if a dll loads in a different location that is not necessarily going to cause a problem.
Yes. I didn't notice you had checked that into CVS (you hadn't yet the time I did look). However, the above does have the advantage of residing in one location rather than 2+.Isn't this already handled in dll_init.cc?d->p = p; } else { + if (in_forkee) + system_printf ("Unexpected dll loaded during fork: %W", name); + /* FIXME: Change this to new at some point. */ d = (dll *) cmalloc (HEAP_2_DLL, sizeof (*d) + (namelen * sizeof (*name)));
@@ -371,7 +378,6 @@ dll_list::load_after_fork (HANDLE parent preferred_block = reserve_at (d->name, (DWORD) h);
} - in_forkee = false; }
struct dllcrt0_info diff --git a/dll_init.h b/dll_init.h --- a/dll_init.h +++ b/dll_init.h @@ -57,7 +57,7 @@ struct dll int init (); void run_dtors () { - if (has_dtors) + if (has_dtors&& !in_forkee) { has_dtors = 0; p.run_dtors ();
Good catch. At one point handle_failure() checked the condition to avoid duplicating code (all callers of fork_api_fatal need to check for in_forkee one way or another), but that seems to have gotten lost in the shuffle.Why is the "in_forkee" dropped here?diff --git a/fork.cc b/fork.cc --- a/fork.cc +++ b/fork.cc @@ -233,6 +233,7 @@ frok::child (volatile char * volatile he sync_with_parent ("loaded dlls", true); }
+ in_forkee = false; init_console_handler (myself->ctty>= 0); ForceCloseHandle1 (fork_info->forker_finished, forker_finished);
@@ -393,10 +394,13 @@ frok::parent (volatile char * volatile s if (!exit_code) continue; this_errno = EAGAIN; - /* Not thread safe, but do we care? */ - __small_sprintf (errbuf, "died waiting for longjmp before initialization, " - "retry %d, exit code %p", ch.retry, exit_code); - error = errbuf; + if (exit_code != EXITCODE_FORK_FAILED) + { + /* Not thread safe, but do we care? */ + __small_sprintf (errbuf, "died waiting for longjmp before initialization, " + "retry %d, exit code %p", ch.retry, exit_code); + error = errbuf; + } goto cleanup; } break; @@ -515,7 +519,8 @@ frok::parent (volatile char * volatile s if (!ch.sync (child->pid, pi.hProcess, FORK_WAIT_TIMEOUT)) { this_errno = EAGAIN; - error = "died waiting for dll loading"; + if (ch.exit_code != EXITCODE_FORK_FAILED) + error = "died waiting for dll loading"; goto cleanup; }
diff --git a/heap.cc b/heap.cc --- a/heap.cc +++ b/heap.cc @@ -88,11 +88,11 @@ heap_init () if ((reserve_size -= page_const)< allocsize) break; } - if (!p&& in_forkee&& !fork_info->handle_failure (GetLastError ())) - api_fatal ("couldn't allocate heap, %E, base %p, top %p, " - "reserve_size %d, allocsize %d, page_const %d", - cygheap->user_heap.base, cygheap->user_heap.top, - reserve_size, allocsize, page_const); + if (!p) + fork_api_fatal ("couldn't allocate heap, %E, base %p, top %p, " + "reserve_size %d, allocsize %d, page_const %d", + cygheap->user_heap.base, cygheap->user_heap.top, + reserve_size, allocsize, page_const);
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