Use picolibc instead of newlib-nano

[keith-packard]

 1. Add picolibc bits to linker scripts. Picolibc uses thread local storage
    for unshared data, and the picolibc startup code relies on carefully
    constructed linker script that allocates one TLS block in RAM for
    applications that don't have explicit TLS support. The picolibc
    startup code also uses different symbols than the tf-m startup code.

 2. Support picolibc stdio. There was already picolibc support code
    present in the library for the ARM LLVM toolchain. The
    conditionals which selected it have been changed to use
    __PICOLIBC__ instead of __clang_major__.

 3. Add _exit stub. Code using assert or abort end up calling _exit
    through the picolibc signal handling code.

 4. Switch to picolibc.specs. This is needed for toolchains which
    don't use picolibc by default, and can also be used without
    trouble in toolchains where picolibc is the default.

[keith-packard]

These are (finally) passing Zephyr tests using both SDK 0.17 and SDK 0.18-alpha4 (although each requires some additional changes which are wending their way upstream). I think they're ready for review and help getting them upstream.

[tomi-font]

How did the linker get confused exactly? Is this really needed?

[keith-packard]

. got set to LOADADDR instead of ADDR when building with binutils 2.43. I'm afraid that after spending several hours chasing this down, I didn't dig into the linker code to figure out precisely why that happened.

[keith-packard]

I've re-added this patch as it is necessary even with SDK 0.17.4.

[tomi-font]

Had a chat with upstream about these (picolibc) changes. Let's keep them only in this fork until they're not needed anymore (TF-M is moving away from C library usage), in the next release or the one following it.
But it would be good to have someone who understands this a bit to review the changes. Don't know who that could be. cc @frkv @d3zd3z @ceolin @dleach02 @Vge0rge

[keith-packard]

Had a chat with upstream about these (picolibc) changes. Let's keep them only in this fork until they're not needed anymore (TF-M is moving away from C library usage), in the next release or the one following it. But it would be good to have someone who understands this a bit to review the changes. Don't know who that could be. cc @frkv @d3zd3z @ceolin @dleach02 @Vge0rge

Sounds good. I'll keep running tests as this series moves forward. This is not buildable using SDK 0.17.2 because of a last-minute oops in that release, but it should be buildable with the upcoming SDK 0.17.3.

[frkv]

I recommend @wearyzen and @adeaarm give it a quick look from TF-M's perspective.

The Clang/LLVM toolchain for TF-M was added with no dependency on libc and/or picolib in the core TF-M code, but you still see references to nano.specs in the GNUARM toolchain

Zephyr, making this choice for GNUARM toolchain (this change), will take any penalty related to support, any security advisories etc... But the choice to go to picolibc is an active choice in the Zephyr community, and in extension it will impact any vendor who makes use of standard c library function in PSA crypto and PSA crypto driver build as well as in bootloaders...

With regards to these commits, I think we should consider them to be marked as commits emitted from zephyr integration, unless they are possible to add upstream in the TF-M project... If they are out-of-tree patches required for Zephyr integration, they would need to be maintained across updated versions of TF-M in our fork

[tomi-font]

With regards to these commits, I think we should consider them to be marked as commits emitted from zephyr integration

Yeah this is something we've agreed on and I think just needs to be implemented (documenting the practice basically). In the meantime I don't think we can require that of anyone yet.

[Anton-TF]

True, TF-M plans to remove libc from all secure binaries (BL1, BL2, tfm_s) for Clang/LLVM and GCC toolchains at least.
As part of this effort, we also intend to clean up and streamline the linker scripts.

Additionally, you might want to consider these recent fixes in the scripts:
https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/40014
https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/40068

[keith-packard]

Sounds like any libc-specific changes will be transient then, which will be nice.

[wearyzen]

While trying to build the non-secure MPS4 board with this PR and Zephyr SDK 0.18.0-alpha4 we see below error:

west build -p -b mps4/corstone320/fvp/ns samples/hello_world/ -t run
...
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: (.text._start+0x30): undefined reference to `__data_source'
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: (.text._start+0x34): undefined reference to `__data_start'
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: (.text._start+0x38): undefined reference to `__bss_size'
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: (.text._start+0x3c): undefined reference to `__bss_start'
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: (.text._start+0x40): undefined reference to `__tls_base'
/home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/bin/ld: /home/zephyr-sdk-0.18.0-alpha4/gnu/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/14.3.0/../../../../arm-zephyr-eabi/lib/thumb/v8-m.main/nofp/space/libc.a(libc_picolib_machine_arm_set_tls.c.o): in function `_set_tls':
set_tls.c:(.text._set_tls+0xc): undefined reference to `__arm32_tls_tcb_offset'

Referring to the changes in this PR, I tried below patch and it seems to fix the issue:

git diff
diff --git a/bl1/bl1_2/bl1_dummy_rotpk.prv b/bl1/bl1_2/bl1_dummy_rotpk.prv
index bf011cd4d..b6bf9b600 100644
Binary files a/bl1/bl1_2/bl1_dummy_rotpk.prv and b/bl1/bl1_2/bl1_dummy_rotpk.prv differ
diff --git a/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_1.ld b/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_1.ld
index 44f04021a..7fad80960 100644
--- a/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_1.ld
+++ b/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_1.ld
@@ -160,6 +160,25 @@ SECTIONS
     } > RAM

     bss_size = __bss_end__ - __bss_start__;
+    /* --- add after the .bss section --- */
+    PROVIDE( __bss_start = __bss_start__ );
+    PROVIDE( __bss_end   = __bss_end__ );
+    PROVIDE( __bss_size  = __bss_end - __bss_start );
+
+    PROVIDE( __data_start  = ADDR(.data) );
+    PROVIDE( __data_end    = __data_end__ );
+    PROVIDE( __data_source = LOADADDR(.data) );
+    PROVIDE( __data_size   = __data_end - __data_start );
+
+    /* BL1_1 doesn’t use TLS, but picolibc’s _set_tls needs these present */
+    PROVIDE( __tls_base = 0 );
+    PROVIDE( __tls_align = 8 );
+    PROVIDE( __arm32_tls_tcb_offset = 8 );
+
+    /* --- add after the .heap section (where __HeapBase/Limit are set) --- */
+    PROVIDE( __heap_start = __HeapBase );
+    PROVIDE( __heap_end   = __HeapLimit );
+

 #if defined (__ARM_FEATURE_CMSE) && (__ARM_FEATURE_CMSE == 3U)
     .msp_stack (NOLOAD) : ALIGN(32)
diff --git a/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_2.ld b/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_2.ld
index 83aaf144f..9151ff3a0 100644
--- a/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_2.ld
+++ b/platform/ext/target/arm/mps4/common/device/source/gcc/mps4_corstone3xx_bl1_2.ld
@@ -162,6 +162,25 @@ SECTIONS

     bss_size = __bss_end__ - __bss_start__;

+    /* --- add after the .bss section --- */
+    PROVIDE( __bss_start = __bss_start__ );
+    PROVIDE( __bss_end   = __bss_end__ );
+    PROVIDE( __bss_size  = __bss_end - __bss_start );
+
+    PROVIDE( __data_start  = ADDR(.data) );
+    PROVIDE( __data_end    = __data_end__ );
+    PROVIDE( __data_source = LOADADDR(.data) );
+    PROVIDE( __data_size   = __data_end - __data_start );
+
+    /* BL1_1 doesn’t use TLS, but picolibc’s _set_tls needs these present */
+    PROVIDE( __tls_base = 0 );
+    PROVIDE( __tls_align = 8 );
+    PROVIDE( __arm32_tls_tcb_offset = 8 );
+
+    /* --- add after the .heap section (where __HeapBase/Limit are set) --- */
+    PROVIDE( __heap_start = __HeapBase );
+    PROVIDE( __heap_end   = __HeapLimit );
+
 #if defined (__ARM_FEATURE_CMSE) && (__ARM_FEATURE_CMSE == 3U)
     .msp_stack (NOLOAD) : ALIGN(32)
     {
(END)

I am not a linker script expert but if above change looks good then please let me know if you would like to include it in this PR.
Thanks!

[tomi-font]

@wearyzen could you review this?

[tejlmand]

this will work for Zephyr SDK, but Zephyr SDK is not the only supported toolchain in Zephyr.

This will not work for users using GNU Arm embedded toolchain with newlib enabled.

Shouldn't we be supporting both options ?

[keith-packard]

We could support both options with some configuration mechanism to select between them, but that seems like a lot of added complexity. For users with the GNU Arm embedded toolchain, we could direct them to download the picolibc add-in and use that for this library?

[wearyzen]

@wearyzen could you review this?

I don't have expertise on toolchain or linker so hard for me to say if there is an issue with this but I remember from my previous comment this change is needed to use with Zephyr SDK 0.18.0-alpha4.