linux-kernel - [PATCH] um: Fix overlapping ELF segments when statically linked

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Message-Id: <20200305183939.256241-1-davidgow@google.com>
Date:   Thu,  5 Mar 2020 10:39:39 -0800
From:   David Gow <davidgow@...gle.com>
To:     Jeff Dike <jdike@...toit.com>, Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>
Cc:     brendanhiggins@...gle.com, trishalfonso@...gle.com,
        linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        David Gow <davidgow@...gle.com>
Subject: [PATCH] um: Fix overlapping ELF segments when statically linked

When statically linked, the .text section in UML kernels is not page
aligned, causing it to share a page with the executable headers. As
.text and the executable headers have different permissions, this causes
the kernel to wish to map the same page twice (once as headers with r--
permissions, once as .text with r-x permissions), causing a segfault,
and a nasty message printed to the host kernel's dmesg:

"Uhuuh, elf segment at 0000000060000000 requested but the memory is
mapped already"

By aligning the .text to a page boundary (as in the dynamically linked
version in dyn.lds.S), there is no such overlap, and the kernel runs
correctly.

Signed-off-by: David Gow <davidgow@...gle.com>
---
I'm not 100% sure what triggered this -- possibly a change to the host
kernel on my machine -- as I'm able to reproduce the issue as far back
as in 4.4, but it seems to be reproducible easily on my machine with
defconfig + CONFIG_STATIC_LINK=y.


 arch/um/kernel/uml.lds.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/um/kernel/uml.lds.S b/arch/um/kernel/uml.lds.S
index 9f21443be2c9..3b6dab3d4501 100644
--- a/arch/um/kernel/uml.lds.S
+++ b/arch/um/kernel/uml.lds.S
@@ -19,10 +19,10 @@ SECTIONS
   __binary_start = START;
 
   . = START + SIZEOF_HEADERS;
+  . = ALIGN(PAGE_SIZE);
 
   _text = .;
   INIT_TEXT_SECTION(0)
-  . = ALIGN(PAGE_SIZE);
 
   .text      :
   {
-- 
2.25.0.265.gbab2e86ba0-goog