feat: add sigOnSigReturn and sigInstallTrampoline to signal

src/signal.c

@@ -315,3 +315,88 @@ SigOutcome sigConsumePending(Thread* thread, Registers* regs, int* outSignum)
         newBlocked &= ~(1ull << sigStop);
         thread->sigBlocked = newBlocked;
 
+        /* Rewrite `regs` in place to enter the shared trampoline: rip is
+           the trampoline's entry, rax is the handler address its
+           `call *%rax` invokes, rdi is signum (the handler's sole SysV
+           argument). regs->rsp (the interrupted user stack) is left
+           untouched -- the trampoline runs on the target's own stack,
+           right where it already was. */
+        regs->rax = handler;
+        regs->rdi = (uint64)signum;
+        regs->rip = sigTrampolineVirtualAddr;
+
+        /* sigFlagSigInfo (signalnum.h): copy the SigInfo sigRaiseInfo
+           attached to this signum into THIS (thread, depth) pair's own
+           slot of the shared sigInfoVirtualAddr page -- a different slot
+           than any still-active outer handler's own siginfo pointer, by
+           the exact same depth-indexed reasoning sigSavedRegs/
+           sigSavedBlocked already use, so a nested sigFlagSigInfo delivery
+           can never clobber an outer one's still-unread copy. rdx is
+           always 0 (no ucontext_t this kernel can expose). Left completely
+           untouched otherwise -- see this function's own doc comment. */
+        if (thread->sigHandlerFlags[signum] & sigFlagSigInfo && sigInfoPageCount > 0) {
+            uint32 slot = thread->id * sigMaxNestDepth + depth;
+            uint32 page = slot / sigInfoSlotsPerPage;
+            uint32 offset = (slot % sigInfoSlotsPerPage) * (uint32)sizeof(SigInfo);
+            SigInfo* dst = (SigInfo*)(sigInfoFrames[page] + offset);
+            *dst = thread->sigPendingInfo[signum];
+            regs->rsi = sigInfoVirtualAddr + (uint64)page * frameSize + offset;
+            regs->rdx = 0;
+        }
+
+        return sigOutcomeHandler;
+    }
+
+    return sigOutcomeNone;
+}
+
+bool sigOnSigReturn(Thread* thread, Registers* regs)
+{
+    if (thread->sigHandlerDepth == 0)
+        return false;
+
+    uint32 depth = thread->sigHandlerDepth - 1;
+    *regs = thread->sigSavedRegs[depth];
+    thread->sigBlocked = thread->sigSavedBlocked[depth];
+    thread->sigHandlerDepth = depth;
+    return true;
+}
+
+void sigInstallTrampoline(void)
+{
+    uint64 frame = pmmAlloc();
+    if (frame == 0)
+        return;
+
+    uint64 length = (uint64)(sigTrampolineEnd - sigTrampolineStart);
+    memset((void*)frame, 0, frameSize);
+    memcpy((void*)frame, sigTrampolineStart, length);
+
+    pagingMap(sigTrampolineVirtualAddr, frame, pagePresent | pageUser);
+}
+
+void sigInstallInfoPage(void)
+{
+    uint64 totalSlots = (uint64)threadCapacity() * sigMaxNestDepth;
+    uint64 pagesNeeded = (totalSlots + sigInfoSlotsPerPage - 1) / sigInfoSlotsPerPage;
+    if (pagesNeeded > sigInfoMaxPages)
+        pagesNeeded = sigInfoMaxPages;
+
+    for (uint32 i = 0; i < pagesNeeded; i++) {
+        uint64 frame = pmmAlloc();
+        if (frame == 0)
+            return;
+
+        memset((void*)frame, 0, frameSize);
+        /* pageUser, no pageWrite: a handler may only ever READ the SigInfo
+           it was handed a pointer to, exactly like the trampoline page
+           above is read+execute, never write -- the kernel is the only
+           writer, via `frame`'s own physical address (see the frame
+           comment above sigInfoFrames), never through this user-facing
+           mapping. */
+        pagingMap(sigInfoVirtualAddr + (uint64)i * frameSize, frame, pagePresent | pageUser);
+        sigInfoFrames[i] = frame;
+    }
+
+    sigInfoPageCount = (uint32)pagesNeeded;
+}