[PATCH] riscv: mmap with PROT_WRITE but no PROT_READ is invalid

[Eva Kotova]

On Tue, 31 May 2022 00:56:52 PDT (-0700), coelacanthus at outlook.com wrote:
 > As mentioned in Table 4.5 in RISC-V spec Volume 2 Section 4.3, write
 > but not read is "Reserved for future use.". For now, they are not valid.
 > In the current code, -wx is marked as invalid, but -w- is not marked
 > as invalid.

This patch breaks OpenJDK/Java on RISC-V, as it tries to create a w-only 
protective page:

#
# There is insufficient memory for the Java Runtime Environment to continue.
# Native memory allocation (mmap) failed to map 4096 bytes for failed to 
allocate memory for PaX check.
# An error report file with more information is saved as:
# /root/hs_err_pid107.log

I bisected to this commit since on Linux 5.19+ java no longer works.
Perhaps some fallback should be implemented, to prevent userspace 
breakage. It is currently documented, that at least on i386 PROT_WRITE 
mappings imply PROT_READ (See man mmap(2) NOTES), this would be a good 
place to start.

Best regards,
Eva