[PowerPC] missing CFI for ELF32 to unwind cr2, cr3, cr4 #83094
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@llvm/issue-subscribers-backend-powerpc Author: George Koehler (kernigh)
For PowerPC ELF 32-bit targets, LLVM forgets to provide CFI like `.cfi_offset cr2, -88` for fields cr2, cr3, cr4 in the condition register. Function calls must preserve cr2, cr3, cr4. Because the CFI is missing, an unwinder (like libunwind) can't restore cr2, cr3, cr4, when it unwinds a function call.
I found the bug after OpenBSD/macppc's clang 16.0.6 miscompiled gdb, when a C++ exception crashed gdb by not restoring cr2. I reproduced the bug in 19.0.0git with this example, struct flip {
int *p;
flip(int *a): p(a) { if (p) *p = 1; }
~flip() { if (p) *p = 0; }
};
#define N __attribute__((noinline))
int i;
N void flip2(int *p) { flip f(p); throw 0; }
N void flip1(int *p) { flip f(p); flip2(&i); }
int main() { try { flip1(nullptr); } catch(int) {} }I compiled it with clang++ -O2. The example is small enough to optimize out cr2; I added I will soon add a pull request. |
kernigh
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Mar 2, 2024
Delete the code that skips the CFI for the condition register on ELF32. The code checked !MustSaveCR, which happened only when Subtarget.is32BitELFABI(), where spillCalleeSavedRegisters is spilling cr in a different way. The spill was missing CFI. After deleting this code, a spill of cr2 to cr4 gets CFI in the same way as a spill of r14 to r31. Fixes llvm#83094
brad0
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Mar 3, 2024
Delete the code that skips the CFI for the condition register on ELF32. The code checked !MustSaveCR, which happened only when Subtarget.is32BitELFABI(), where spillCalleeSavedRegisters is spilling cr in a different way. The spill was missing CFI. After deleting this code, a spill of cr2 to cr4 gets CFI in the same way as a spill of r14 to r31. Fixes #83094
llvmbot
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Mar 23, 2024
Delete the code that skips the CFI for the condition register on ELF32. The code checked !MustSaveCR, which happened only when Subtarget.is32BitELFABI(), where spillCalleeSavedRegisters is spilling cr in a different way. The spill was missing CFI. After deleting this code, a spill of cr2 to cr4 gets CFI in the same way as a spill of r14 to r31. Fixes llvm#83094 (cherry picked from commit 6b70c5d)
tstellar
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Mar 27, 2024
Delete the code that skips the CFI for the condition register on ELF32. The code checked !MustSaveCR, which happened only when Subtarget.is32BitELFABI(), where spillCalleeSavedRegisters is spilling cr in a different way. The spill was missing CFI. After deleting this code, a spill of cr2 to cr4 gets CFI in the same way as a spill of r14 to r31. Fixes llvm#83094 (cherry picked from commit 6b70c5d)
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For PowerPC ELF 32-bit targets, LLVM forgets to provide CFI like
.cfi_offset cr2, -88for fields cr2, cr3, cr4 in the condition register. Function calls must preserve cr2, cr3, cr4. Because the CFI is missing, an unwinder (like libunwind) can't restore cr2, cr3, cr4, when it unwinds a function call.I found the bug after OpenBSD/macppc's clang 16.0.6 miscompiled gdb, when a C++ exception crashed gdb by not restoring cr2. I reproduced the bug in 19.0.0git with this example,
I compiled it with clang++ -O2. The example is small enough to optimize out cr2; I added
noinlineto keep cr2 in the code. The functions flip1 and flip2 each use cr2 to test some pointer p, which is null in flip1 and non-null in flip2. The C++ exception fromthrow 0unwinds flip2 without restoring cr2, then runs the C++ destructor ~flip in flip1 with a wrong cr2, soif (p) *p = 0crashes by writing to a null p.I will soon add a pull request.
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