ARM: 9252/1: module: Teach unwinder about PLTs

"unwind: Index not found eef26358" warnings keep popping up on
CONFIG_ARM_MODULE_PLTS-enabled systems if the PC points to a PLT veneer.
Teach the unwinder how to deal with them, taking into account they don't
change state of the stack or register file except loading PC.

Link: https://lore.kernel.org/linux-arm-kernel/20200402153845.30985-1-kursad.oney@broadcom.com/Tested-by: Kursad Oney <kursad.oney@broadcom.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/include/asm/module.h

@@ -37,6 +37,11 @@ struct mod_arch_specific {
 
 struct module;
 u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val);
+#ifdef CONFIG_ARM_MODULE_PLTS
+bool in_module_plt(unsigned long loc);
+#else
+static inline bool in_module_plt(unsigned long loc) { return false; }
+#endif
 
 #ifdef CONFIG_THUMB2_KERNEL
 #define HAVE_ARCH_KALLSYMS_SYMBOL_VALUE

arch/arm/kernel/module-plts.c

@@ -284,3 +284,17 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 		 mod->arch.core.plt->sh_size, mod->arch.init.plt->sh_size);
 	return 0;
 }
+
+bool in_module_plt(unsigned long loc)
+{
+	struct module *mod;
+	bool ret;
+
+	preempt_disable();
+	mod = __module_text_address(loc);
+	ret = mod && (loc - (u32)mod->arch.core.plt_ent < mod->arch.core.plt_count * PLT_ENT_SIZE ||
+		      loc - (u32)mod->arch.init.plt_ent < mod->arch.init.plt_count * PLT_ENT_SIZE);
+	preempt_enable();
+
+	return ret;
+}

arch/arm/kernel/unwind.c

@@ -28,6 +28,7 @@
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/list.h>
+#include <linux/module.h>
 
 #include <asm/stacktrace.h>
 #include <asm/traps.h>
@@ -395,8 +396,18 @@ int unwind_frame(struct stackframe *frame)
 
 	idx = unwind_find_idx(frame->pc);
 	if (!idx) {
-		if (frame->pc && kernel_text_address(frame->pc))
+		if (frame->pc && kernel_text_address(frame->pc)) {
+			if (in_module_plt(frame->pc) && frame->pc != frame->lr) {
+				/*
+				 * Quoting Ard: Veneers only set PC using a
+				 * PC+immediate LDR, and so they don't affect
+				 * the state of the stack or the register file
+				 */
+				frame->pc = frame->lr;
+				return URC_OK;
+			}
 			pr_warn("unwind: Index not found %08lx\n", frame->pc);
+		}
 		return -URC_FAILURE;
 	}