Add the .sdata section to the app binary.
RISC-V has the .sdata section, which is supposed to be for commonly-accessed global variables. Previously, we discarded this section in the linker script. Discarding this section resulted in missing global variables.
I originally discovered this when I tried to print a string of less than 9 bytes and it was not working. I reported it as a kernel bug at https://github.com/tock/tock/issues/2132, then later realized the issue was in the libtock-rs linker script rather than the kernel.
I added a test to libtock_test that catches this issue (I verified by hand that the test fails if I don't fix the linker script).
diff --git a/examples-features/libtock_test.rs b/examples-features/libtock_test.rs
index ac2cb48..a938f46 100644
--- a/examples-features/libtock_test.rs
+++ b/examples-features/libtock_test.rs
@@ -10,6 +10,7 @@
use core::future::Future;
use core::mem;
use core::pin::Pin;
+use core::ptr::read_volatile;
use core::task::Context;
use core::task::Poll;
use libtock::console::ConsoleDriver;
@@ -46,6 +47,7 @@
gpio: &mut GpioDriverFactory,
) -> TockResult<()> {
test.console()?;
+ test.sdata()?;
test.static_mut()?;
test.dynamic_dispatch()?;
test.formatting()?;
@@ -72,6 +74,21 @@
self.log_success("Console")
}
+ // RISC-V has the .sdata section, which is ostensibly for commonly-accessed
+ // read-write global variables. In practice, the Rust compiler tends to put
+ // small globals (<= 8 bytes) into it. This test verifies that small
+ // data symbols are accessible.
+ fn sdata(&mut self) -> TockResult<()> {
+ static mut SDATA: [u8; 4] = [1, 2, 3, 4];
+ // Use volatile reads to prevent the compiler from constant-folding
+ // everything away.
+ let sdata_ptr = unsafe { &SDATA } as *const u8;
+ let sum = (0..4)
+ .map(|i| unsafe { read_volatile(sdata_ptr.offset(i)) })
+ .sum::<u8>();
+ self.check_if_true(sum == 10, "sdata")
+ }
+
fn static_mut(&mut self) -> TockResult<()> {
self.check_if_true(increment_static_mut() == 1, "static mut")
}
diff --git a/layout_generic.ld b/layout_generic.ld
index dd101ab..3c5065c 100644
--- a/layout_generic.ld
+++ b/layout_generic.ld
@@ -113,6 +113,7 @@
. = ALIGN(4); /* Make sure we're word-aligned here */
_data = .;
KEEP(*(.data*))
+ *(.sdata*) /* RISC-V small-pointer data section */
. = ALIGN(4); /* Make sure we're word-aligned at the end of flash */
} > SRAM
