uml: fix FP register corruption

Commit ee3d9bd4de1ed93d2a7ee41c331ed30a1c7b8acd ("uml: simplify SIGSEGV
handling"), while greatly simplifying the kernel SIGSEGV handler that
runs in the process address space, introduced a bug which corrupts FP
state in the process.

Previously, the SIGSEGV handler called the sigreturn system call by hand - it
couldn't return through the restorer provided to it because that could try to
call the libc restorer which likely wouldn't exist in the process address
space.  So, it blocked off some signals, including SIGUSR1, on entry to the
SIGSEGV handler, queued a SIGUSR1 to itself, and invoked sigreturn.  The
SIGUSR1 was delivered, and was visible to the UML kernel after sigreturn
finished.

The commit eliminated the signal masking and the call to sigreturn.  The
handler simply hits itself with a SIGTRAP to let the UML kernel know that it
is finished.  UML then restores the process registers, which effectively
longjmps the process out of the signal handler, skipping sigreturn's restoring
of register state and the signal mask.

The bug is that the host apparently sets used_fp to 0 when it saves the
process FP state in the sigcontext on the process signal stack.  Thus, when
the process is longjmped out of the handler, its FP state is corrupt because
it wasn't saved on the context switch to the UML kernel.

This manifested itself as sleep hanging.  For some reason, sleep uses floating
point in order to calculate the sleep interval.  When a page fault corrupts
its FP state, it is faked into essentially sleeping forever.

This patch saves the FP state before entering the SIGSEGV handler and restores
it afterwards.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/um/include/registers.h b/arch/um/include/registers.h
index 9ea1ae3c8f464d8c8b138eecf8d3694f238d32c5..b0b4589e0ebce804c3a6189da1835620069410cf 100644 (file)
--- a/arch/um/include/registers.h
+++ b/arch/um/include/registers.h
@@ -18,5 +18,7 @@ extern int restore_registers(int pid, struct uml_pt_regs *regs);
 extern int init_registers(int pid);
 extern void get_safe_registers(unsigned long *regs);
 extern unsigned long get_thread_reg(int reg, jmp_buf *buf);
+extern int get_fp_registers(int pid, unsigned long *regs);
+extern int put_fp_registers(int pid, unsigned long *regs);
 
 #endif

diff --git a/arch/um/include/sysdep-i386/ptrace_user.h b/arch/um/include/sysdep-i386/ptrace_user.h
index 899aa4b2a78d542888d09a4812fbb13da34035e3..75650723c38f15506144d12f5cfedfb0f63a8072 100644 (file)
--- a/arch/um/include/sysdep-i386/ptrace_user.h
+++ b/arch/um/include/sysdep-i386/ptrace_user.h
@@ -9,6 +9,7 @@
 #include <sys/ptrace.h>
 #include <linux/ptrace.h>
 #include <asm/ptrace.h>
+#include "user_constants.h"
 
 #define PT_OFFSET(r) ((r) * sizeof(long))
 
@@ -40,6 +41,8 @@
 #define PT_SP_OFFSET PT_OFFSET(UESP)
 #define PT_SP(regs) ((regs)[UESP])
 
+#define FP_SIZE ((HOST_XFP_SIZE > HOST_FP_SIZE) ? HOST_XFP_SIZE : HOST_FP_SIZE)
+
 #ifndef FRAME_SIZE
 #define FRAME_SIZE (17)
 #endif

diff --git a/arch/um/include/sysdep-x86_64/ptrace_user.h b/arch/um/include/sysdep-x86_64/ptrace_user.h
index 4cd61a852fabd5c7af23beb1cab016cbff3ff393..45c0bd881cb3a36b64b657ee9090ee8a0bdf3b5a 100644 (file)
--- a/arch/um/include/sysdep-x86_64/ptrace_user.h
+++ b/arch/um/include/sysdep-x86_64/ptrace_user.h
@@ -12,6 +12,7 @@
 #include <linux/ptrace.h>
 #include <asm/ptrace.h>
 #undef __FRAME_OFFSETS
+#include "user_constants.h"
 
 #define PT_INDEX(off) ((off) / sizeof(unsigned long))
 
@@ -69,6 +70,8 @@
 #define REGS_IP_INDEX PT_INDEX(RIP)
 #define REGS_SP_INDEX PT_INDEX(RSP)
 
+#define FP_SIZE (HOST_FP_SIZE)
+
 #endif
 
 /*

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index b14829469faefdf99a2511beede3ea6515c21927..1e8cba6550a99824df296e468747fabc109d6d44 100644 (file)
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -115,6 +115,14 @@ void get_skas_faultinfo(int pid, struct faultinfo * fi)
                               sizeof(struct ptrace_faultinfo));
        }
        else {
+               unsigned long fpregs[FP_SIZE];
+
+               err = get_fp_registers(pid, fpregs);
+               if (err < 0) {
+                       printk(UM_KERN_ERR "save_fp_registers returned %d\n",
+                              err);
+                       fatal_sigsegv();
+               }
                err = ptrace(PTRACE_CONT, pid, 0, SIGSEGV);
                if (err) {
                        printk(UM_KERN_ERR "Failed to continue stub, pid = %d, "
@@ -128,6 +136,13 @@ void get_skas_faultinfo(int pid, struct faultinfo * fi)
                 * the stub stack page. We just have to copy it.
                 */
                memcpy(fi, (void *)current_stub_stack(), sizeof(*fi));
+
+               err = put_fp_registers(pid, fpregs);
+               if (err < 0) {
+                       printk(UM_KERN_ERR "put_fp_registers returned %d\n",
+                              err);
+                       fatal_sigsegv();
+               }
        }
 }
 

diff --git a/arch/um/os-Linux/sys-i386/registers.c b/arch/um/os-Linux/sys-i386/registers.c
index f74d853a0ee02372af46d4ab4ae244242a19ea4f..b613473b3ec1c1ef9ba83da234a207ef6f08aa38 100644 (file)
--- a/arch/um/os-Linux/sys-i386/registers.c
+++ b/arch/um/os-Linux/sys-i386/registers.c
@@ -56,6 +56,22 @@ unsigned long get_thread_reg(int reg, jmp_buf *buf)
 
 int have_fpx_regs = 1;
 
+int get_fp_registers(int pid, unsigned long *regs)
+{
+       if (have_fpx_regs)
+               return save_fpx_registers(pid, regs);
+       else
+               return save_fp_registers(pid, regs);
+}
+
+int put_fp_registers(int pid, unsigned long *regs)
+{
+       if (have_fpx_regs)
+               return restore_fpx_registers(pid, regs);
+       else
+               return restore_fp_registers(pid, regs);
+}
+
 void arch_init_registers(int pid)
 {
        unsigned long fpx_regs[HOST_XFP_SIZE];

diff --git a/arch/um/os-Linux/sys-x86_64/registers.c b/arch/um/os-Linux/sys-x86_64/registers.c
index a375853337a703d7764dded31549725cf17376e8..594d97ad02b3408b8b351a3b4263f82cb88461ad 100644 (file)
--- a/arch/um/os-Linux/sys-x86_64/registers.c
+++ b/arch/um/os-Linux/sys-x86_64/registers.c
@@ -40,3 +40,13 @@ unsigned long get_thread_reg(int reg, jmp_buf *buf)
                return 0;
        }
 }
+
+int get_fp_registers(int pid, unsigned long *regs)
+{
+       return save_fp_registers(pid, regs);
+}
+
+int put_fp_registers(int pid, unsigned long *regs)
+{
+       return restore_fp_registers(pid, regs);
+}