Details
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Bug
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Resolution: Fixed
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P4
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19, 20
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b04
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aarch32
Backports
Issue | Fix Version | Assignee | Priority | Status | Resolution | Resolved In Build |
---|---|---|---|---|---|---|
JDK-8312101 | 17.0.9 | Sergey Nazarkin | P4 | Resolved | Fixed | b01 |
Description
This was only reproducible if the VM was built natively, on a Raspberry Pi, inside an Ubuntu18-derived container. Buiding natively on Raspberry Pi OS was fine. Cross-building was fine too. The difference is the default instruction set the toolchain uses. We don't explicitly specify `-mthumb` or `-marm`, so we use the toolchain's default. That default seems to depend on how GCC itself was built. Ubuntu ships a GCC that has been built in thumb mode, thus defaulting to `-mthumb`, whereas Rasperry Pi OS and Fedora ship GCCs that default to `-marm`.
So, the VM proper is compiled either to arm or thumb code. The `SafeFetch32` assembly function itself uses arm code always. Why this is I don't know for sure, I assume if I wanted thumb I need to specify `.thumb_func` in the assembly.
If the VM uses thumb, it needs to call SafeFetch32 with a switching branch instruction (BX). But the compiler-generated BL. The instruction set was not switched upon entering SafeFetch32 and garbage thumb code was executed. VM crashes soon after.
This seems to be a common problem when writing arm assembly by hand, the solution is specify `.type function`. See also [1]: "As of GCC 4.7, the .type directive is pretty much required for functions. Or, rather, it is required if you want ARM and Thumb interworking to work."
A remaining question is whether we should specify the instruction set explicitly when building on arm32, to prevent surprises like this. Preferably with a configure option.
Attachments
Issue Links
- backported by
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JDK-8312101 [arm32] SafeFetch32 thumb interleaving causes random crashes
- Resolved
- relates to
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JDK-8313790 [arm32] Specify -marm when building without an ABI profile
- In Progress
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JDK-8284997 arm32 build crashes since JDK-8283326
- Resolved
- links to
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Commit openjdk/jdk17u-dev/8c1a5bcd
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Commit openjdk/jdk/26c03c18
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Review openjdk/jdk17u-dev/1577
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Review openjdk/jdk/9213
(2 links to)