Bug 180351

From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.12) Gecko/20050922 Fedora/1.0.7-1.1.fc4 Firefox/1.0.7

Description of problem:
The way that the kernel handles rt_sigframe, and the unwind information for rt_sigreturn in the kernel-supplied vDSO, inhibits user virtualization of signal handling.  The rt_sigframe becomes a large block that must be preserved, which breaks existing virtualizations that move only the four scalars {pretcode, sig, pinfo, puc}.  The most common symptom is SIGSEGV in thread kill during pthread_cond_wait or when returning from signal handler.  The attached patch allows such user virtualizations.

First, the patch corrects the declaration of puc in arch/i386/kernel/sigframe.h to be "struct ucontext __user *" instead of "void __user *",  and adds a comment that the pointers at the beginning of struct rt_sigframe should be used to access the structures at the end.  Although rt_sigframe is how the kernel constructs the frame, all accesses should be indirect through the pointers in the initial part.  Second, the patch changes the unwind information in arch/i386/kernel/vsyscall-sigreturn.S to use the pointers instead of accessing the structs directly.  There is one more indirection, which costs 20 bytes: two bytes times ten items.  Each item now has three more elements, but an offset is less than 128, so is shorter in the leb128 encoding.  These first two changes allow signals to be unwound successfully by the code in ld-linux.so from glibc-2.3.90, even if the virtualized handler moves only {pretcode, sig, pinfo, puc}.  Third, the patch changes arch/i386/kernel/signal.c so that rt_sigreturn uses the pointers at the beginning of rt_sigframe.  This allows successful return from signal when user virtualization moves the pointers at the beginning of rt_sigframe.  Strictly speaking, the declaration of user signal handler must add 'const' to prevent the compiler from changing the pointers:
  void handler(int sig, struct siginfo *info, struct ucontext const *puc)
In practice current compilers do not require such a change yet, and any potential impact is also mitigated by scant user documentation which results in low usage.

Version-Release number of selected component (if applicable):
2.6.15-1.1826.2.10_FC5 and later

How reproducible:
Always

Steps to Reproduce:
1. Receive a signal when SA_SIGINFO requires using struct rt_sigframe.
2. Call an inner handler with a new virtualized frame using only the {pretcode, sig, pinfo, puc} at the beginning of rt_sigframe.
3. Unwind past the signal frame, or return directly to the kernel from the inner virtualized frame.
  

Actual Results:  SIGSEGV.

Expected Results:  Successful unwind or return from signal.

Additional info: