RFR: 8284273: Early crashes in os::print_context on AArch64

[Gerard Ziemski]

Our error reporting mechanism can crash again, while reporting the original crash, because we try to access a possibly bogus memory location, for example while printing a location of memory referred by a CPU register. In such case, we skip over the already reported section (including the one where the secondary crash has occurred).

This is possible, because we section the error reporting code into sections, called "steps", with a state that retains the information about the last attempted step.

Here, we are concerned with two particular steps: "printing register info" and "printing registers, top of stack, instructions near pc". In those two steps we report 4 distinct chunks of info:

1. registers' raw values (safe)
2. registers' decoded content (risky)
3. memory around the "sp" (risky)
4. memory around the "pc" (risky)

The issue here is that on some platforms (Linux aarch64) a single "step" contains 2, 3 and 4, all "risky" sections, so if we crash early in the step, we skip the rest of it and end up never reporting the later sections, ex:


STEP("printing register info")
  print_register_info
    1. registers' raw values (safe)

STEP("printing registers, top of stack, instructions near pc")
  print_context
    2. registers' decoded content (risky)
    3. memory around the "sp" (risky)
    4. memory around the "pc" (risky)


Other platforms (Linux x64) have a single "step" containing 1, 3 and 4, and another one containing just 2, so that we always get to report section 3, ex:


STEP("printing register info")
  print_register_info
    2. registers' decoded content (risky)

STEP("printing registers, top of stack, instructions near pc")
  print_context
    1. registers' raw values (safe)
    3. memory around the "sp" (risky)
    4. memory around the "pc" (risky)


This fix proposes to rearrange the STEPS, so that the less risky sections come first and to split the STEP with 3 sections into 2 STEPS, like  so:


STEP("printing registers")
  print_context
    1. registers' raw values (safe)

STEP("printing register info")
  print_register_info
    2. registers' decoded content (risky)

STEP("printing top of stack, instructions near pc")
  print_tos_pc
    3. memory around the "sp" (risky)
    4. memory around the "pc" (risky)

-------------

Commit messages:
 - move risky sections into separate STEPs

Changes: https://git.openjdk.java.net/jdk/pull/8736/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8736&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284273
  Stats: 124 lines in 15 files changed: 95 ins; 11 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8736.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8736/head:pull/8736

PR: https://git.openjdk.java.net/jdk/pull/8736