um: mark rodata read-only and implement _nofault accesses

Mark read-only data actually read-only (simple mprotect), and
to be able to test it also implement _nofault accesses. This
works by setting up a new "segv_continue" pointer in current,
and then when we hit a segfault we change the signal return
context so that we continue at that address. The code using
this sets it up so that it jumps to a label and then aborts
the access that way, returning -EFAULT.

It's possible to optimize the ___backtrack_faulted() thing by
using asm goto (compiler version dependent) and/or gcc's (not
sure if clang has it) &&label extension, but at least in one
attempt I made the && caused the compiler to not load -EFAULT
into the register in case of jumping to the &&label from the
fault handler. So leave it like this for now.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Co-developed-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20250210160926.420133-2-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

4 files changed, 13 insertions, 6 deletions

diff --git a/arch/um/include/shared/arch.h b/arch/um/include/shared/arch.h
index 880ee42a3329..cc398a21ad96 100644
--- a/arch/um/include/shared/arch.h
+++ b/arch/um/include/shared/arch.h
@@ -12,4 +12,6 @@ extern void arch_check_bugs(void);
 extern int arch_fixup(unsigned long address, struct uml_pt_regs *regs);
 extern void arch_examine_signal(int sig, struct uml_pt_regs *regs);
 
+void mc_set_rip(void *_mc, void *target);
+
 #endif
diff --git a/arch/um/include/shared/as-layout.h b/arch/um/include/shared/as-layout.h
index ea65f151bf48..4f44dcce8a7c 100644
--- a/arch/um/include/shared/as-layout.h
+++ b/arch/um/include/shared/as-layout.h
@@ -50,7 +50,7 @@ extern int linux_main(int argc, char **argv, char **envp);
 extern void uml_finishsetup(void);
 
 struct siginfo;
-extern void (*sig_info[])(int, struct siginfo *si, struct uml_pt_regs *);
+extern void (*sig_info[])(int, struct siginfo *si, struct uml_pt_regs *, void *);
 
 #endif
 
diff --git a/arch/um/include/shared/irq_user.h b/arch/um/include/shared/irq_user.h
index da0f6eea30d0..88835b52ae2b 100644
--- a/arch/um/include/shared/irq_user.h
+++ b/arch/um/include/shared/irq_user.h
@@ -15,7 +15,8 @@ enum um_irq_type {
 };
 
 struct siginfo;
-extern void sigio_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs);
+extern void sigio_handler(int sig, struct siginfo *unused_si,
+			  struct uml_pt_regs *regs, void *mc);
 void sigio_run_timetravel_handlers(void);
 extern void free_irq_by_fd(int fd);
 extern void deactivate_fd(int fd, int irqnum);
diff --git a/arch/um/include/shared/kern_util.h b/arch/um/include/shared/kern_util.h
index f21dc8517538..00ca3e12fd9a 100644
--- a/arch/um/include/shared/kern_util.h
+++ b/arch/um/include/shared/kern_util.h
@@ -24,10 +24,12 @@ extern void free_stack(unsigned long stack, int order);
 struct pt_regs;
 extern void do_signal(struct pt_regs *regs);
 extern void interrupt_end(void);
-extern void relay_signal(int sig, struct siginfo *si, struct uml_pt_regs *regs);
+extern void relay_signal(int sig, struct siginfo *si, struct uml_pt_regs *regs,
+			 void *mc);
 
 extern unsigned long segv(struct faultinfo fi, unsigned long ip,
-			  int is_user, struct uml_pt_regs *regs);
+			  int is_user, struct uml_pt_regs *regs,
+			  void *mc);
 extern int handle_page_fault(unsigned long address, unsigned long ip,
 			     int is_write, int is_user, int *code_out);
 
@@ -59,8 +61,10 @@ extern unsigned long from_irq_stack(int nested);
 
 extern int singlestepping(void);
 
-extern void segv_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs);
-extern void winch(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs);
+extern void segv_handler(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
+			 void *mc);
+extern void winch(int sig, struct siginfo *unused_si, struct uml_pt_regs *regs,
+		  void *mc);
 extern void fatal_sigsegv(void) __attribute__ ((noreturn));
 
 void um_idle_sleep(void);