[klibc] [PATCH klibc 3/3] riscv64: Move shared library below executables

Ben Hutchings ben at decadent.org.uk
Sun Jul 16 09:12:57 PDT 2023

The default base address for riscv64 executables is 64 kiB, and I
originally selected a base address for the shared library of 2 MiB.
This limits the size of executables using it to be < 2 MiB, and it
also interacts with a bug in QEMU's user-mode emulation, causing
executables to immediately crash.  It doesn't help code generation,
because single-instruction jumps have a range of only ±1 MiB and
two-instruction jumps have a range of ±2 GiB.

Move the shared library to 64 kiB and executables to 576 kiB, to allow
for generation of single-instruction jumps.  (klibc.so is currently
much smaller than the 512 kiB this allows for.)

References: https://bugs.debian.org/1040981
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
 usr/klibc/arch/riscv64/MCONFIG | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/usr/klibc/arch/riscv64/MCONFIG b/usr/klibc/arch/riscv64/MCONFIG
index 34061086..717aedaa 100644
--- a/usr/klibc/arch/riscv64/MCONFIG
+++ b/usr/klibc/arch/riscv64/MCONFIG
@@ -13,8 +13,13 @@ KLIBCOPTFLAGS     += -g
-# Normal binaries start at 64 KB, so start the libary at 2 MB.
+# Normal binaries start at 64 kiB. Jumps can use either a single
+# instruction with offset of ±1 MiB, or two instructions with offset
+# of ±2 GiB. Putting klibc.so close above the executable can cause
+# breakage, so instead swap them around: klibc.so at 64 kiB and
+# executable at 576 kiB.
 KLIBCSHAREDFLAGS  += --defsym '__global_pointer$$=0'
 # Kernel has never used stack trampolines
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