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[riscv] Perform a writability test before applying relocations

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If paging is not supported, then we will attempt to apply dynamic
relocations to fix up the runtime addresses.  If the image is
currently executing directly from flash memory, this can result in
effectively sending an undefined sequence of commands to the flash
device, which can cause unwanted side effects.

Perform an explicit writability test before applying relocations,
using a write value chosen to be safe for at least any devices
conforming to the JEDEC Common Flash Interface (CFI01).

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-05-13 17:36:53 +01:00
parent 4566f59757
commit 6fd927f929
1 changed files with 37 additions and 8 deletions
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45
src/arch/riscv/prefix/libprefix.S
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@@ -280,6 +280,23 @@ apply_relocs:
beqz a2, apply_relocs_done
progress " reloc"
/* Test writability
*
* We do this to avoid accidentally sending an undefined
* sequence of commands to a flash device, if we are started
* from read-only memory with no paging support.
*
* We attempt to write an all-ones pattern, on the basis that
* this pattern will harmlessly cause any flash device
* conforming to the CFI01 specification to enter the default
* "read array" state.
*/
la t0, apply_relocs_test
li t1, -1
STOREN t1, (t0)
LOADN t2, (t0)
bne t1, t2, apply_relocs_failed
apply_relocs_loop:
/* Read new relocation record */
LOADN a3, (a0)
@@ -307,23 +324,35 @@ apply_relocs_loop:
/* Loop until we have reached a terminator record (MSB=0, offset=0) */
bnez a3, apply_relocs_loop
/* Check that relocations were applied successfully
*
* Failure to apply relocations (if relocations were needed)
* is a fatal error.
*/
/* Check that relocations were applied successfully */
la t0, _prefix
LOADN t1, prefix_virt
beq t0, t1, apply_relocs_done
progress " failed\n"
j reset_system
bne t0, t1, apply_relocs_failed
apply_relocs_done:
/* Return to caller */
progress " ok\n"
ret
apply_relocs_failed:
/* Failure to apply relocations (if relocations were needed)
* is a fatal error.
*/
progress " failed\n"
j reset_system
.size apply_relocs, . - apply_relocs
/* Writability test
*
* Placed within .data rather than .bss, since we need this to
* be within the range of the stored iPXE image.
*/
.section ".data.apply_relocs_test", "aw", @progbits
.balign ( __riscv_xlen / 8 )
apply_relocs_test:
.space ( __riscv_xlen / 8 )
.size apply_relocs_test, . - apply_relocs_test
/*****************************************************************************
*
* Enable paging