[U-Boot] [PATCH] Makefile: ensure DTB doesn't overflow into initial stack

[Tom Warren]

Stephen,

-----Original Message-----
From: Stephen Warren [mailto:swarren at wwwdotorg.org] 
Sent: Tuesday, January 9, 2018 12:52 PM
To: u-boot at lists.denx.de; Simon Glass <sjg at chromium.org>; Tom Warren <TWarren at nvidia.com>; Stephen Warren <swarren at nvidia.com>
Cc: Tom Rini <trini at konsulko.com>
Subject: [PATCH] Makefile: ensure DTB doesn't overflow into initial stack

From: Stephen Warren <swarren at nvidia.com>

With CONFIG_SYS_INIT_SP_BSS_OFFSET enabled, the initial (pre-relocation) stack is placed some distance after bss_start. The control DTB is appended to the U-Boot binary at bss_start. If the DTB is too large, or the SP BSS offset too small, then the initial stack could corrupt the DTB. Enhance the Makefile to check whether this is likely to occur.

Signed-off-by: Stephen Warren <swarren at nvidia.com>
---
This builds on top of my previous patch "ARMv8: Allow dynamic early stack pointer". However, since all the logic is conditional and only activated if CONFIG_SYS_INIT_SP_BSS_OFFSET is defined, it can be applied with or without that other patch. It'd make sense to apply it afterwards and in the same branch though, or the change won't make a lot of sense to someone reading history in order.

I've got this in my TOT u-boot-tegra/master, ready to send a PR to TomR.  But it's delegated to him in Patchwork.  Do you want me to remove it from my PR, or leave it in?

Tom

 Makefile | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/Makefile b/Makefile
index d8f419bcd900..52cd6ea72161 100644
--- a/Makefile
+++ b/Makefile
@@ -811,6 +811,10 @@ ifneq ($(CONFIG_BUILD_TARGET),)  ALL-y += $(CONFIG_BUILD_TARGET:"%"=%)  endif
 
+ifneq ($(CONFIG_SYS_INIT_SP_BSS_OFFSET),)
+ALL-y += init_sp_bss_offset_check
+endif
+
 LDFLAGS_u-boot += $(LDFLAGS_FINAL)
 
 # Avoid 'Not enough room for program headers' error on binutils 2.28 onwards.
@@ -939,6 +943,33 @@ binary_size_check: u-boot-nodtb.bin FORCE
 		fi \
 	fi
 
+ifneq ($(CONFIG_SYS_INIT_SP_BSS_OFFSET),)
+ifneq ($(CONFIG_SYS_MALLOC_F_LEN),)
+subtract_sys_malloc_f_len = space=$$(($${space} - 
+$(CONFIG_SYS_MALLOC_F_LEN))) else subtract_sys_malloc_f_len = true 
+endif # The 1/4 margin below is somewhat arbitrary. The likely initial 
+SP usage is # so low that the DTB could probably use 90%+ of the 
+available space, for # current values of CONFIG_SYS_INIT_SP_BSS_OFFSET 
+at least. However, let's be # safe for now and tweak this later if 
+space becomes tight.
+# A rejected alternative would be to check that some absolute minimum 
+stack # space was available. However, since 
+CONFIG_SYS_INIT_SP_BSS_OFFSET is # deliberately build-specific, to take 
+account of build-to-build stack usage # differences due to different 
+feature sets, there is no common absolute value # to check against.
+init_sp_bss_offset_check: u-boot.dtb FORCE
+	@dtb_size=$(shell wc -c u-boot.dtb | awk '{print $$1}') ; \
+	space=$(CONFIG_SYS_INIT_SP_BSS_OFFSET) ; \
+	$(subtract_sys_malloc_f_len) ; \
+	quarter_space=$$(($${space} / 4)) ; \
+	if [ $${dtb_size} -gt $${quarter_space} ]; then \
+		echo "u-boot.dtb is larger than 1 quarter of " >&2 ; \
+		echo "(CONFIG_SYS_INIT_SP_BSS_OFFSET - CONFIG_SYS_MALLOC_F_LEN)" >&2 ; \
+		exit 1 ; \
+	fi
+endif
+
 u-boot-nodtb.bin: u-boot FORCE
 	$(call if_changed,objcopy)
 	$(call DO_STATIC_RELA,$<,$@,$(CONFIG_SYS_TEXT_BASE))
--
2.15.1
--
nvpublic