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commit 15eab1e3e89129ab3ed03f5bdc3415b26e9caeb9
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Author: H.J. Lu <hjl.tools@gmail.com>
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Date: Sat Feb 1 05:44:55 2020 -0800
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i386: Don't unnecessarily save and restore EAX, ECX and EDX [BZ# 25262]
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On i386, since EAX, ECX and EDX are caller-saved, there are no need
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to save and restore EAX, ECX and EDX in getcontext, setcontext and
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swapcontext. They just need to clear EAX on success. The extra
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scratch registers are needed to enable CET.
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Tested on i386.
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Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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---
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diff --git a/sysdeps/unix/sysv/linux/i386/getcontext.S b/sysdeps/unix/sysv/linux/i386/getcontext.S
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index 26ca08a..6637596 100644
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--- a/sysdeps/unix/sysv/linux/i386/getcontext.S
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+++ b/sysdeps/unix/sysv/linux/i386/getcontext.S
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@@ -26,13 +26,7 @@ ENTRY(__getcontext)
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/* Load address of the context data structure. */
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movl 4(%esp), %eax
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- /* Return value of getcontext. EAX is the only register whose
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- value is not preserved. */
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- movl $0, oEAX(%eax)
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-
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- /* Save the 32-bit register values and the return address. */
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- movl %ecx, oECX(%eax)
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- movl %edx, oEDX(%eax)
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+ /* Save the preserved register values and the return address. */
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movl %edi, oEDI(%eax)
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movl %esi, oESI(%eax)
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movl %ebp, oEBP(%eax)
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diff --git a/sysdeps/unix/sysv/linux/i386/setcontext.S b/sysdeps/unix/sysv/linux/i386/setcontext.S
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index a604fca..7565d7d 100644
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--- a/sysdeps/unix/sysv/linux/i386/setcontext.S
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+++ b/sysdeps/unix/sysv/linux/i386/setcontext.S
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@@ -65,22 +65,19 @@ ENTRY(__setcontext)
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cfi_offset (esi, oESI)
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cfi_offset (ebp, oEBP)
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cfi_offset (ebx, oEBX)
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- cfi_offset (edx, oEDX)
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- cfi_offset (ecx, oECX)
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movl oESP(%eax), %esp
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/* Push the return address on the new stack so we can return there. */
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pushl %ecx
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- /* Load the values of all the 32-bit registers (except ESP).
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- Since we are loading from EAX, it must be last. */
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+ /* Load the values of all the preserved registers (except ESP). */
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movl oEDI(%eax), %edi
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movl oESI(%eax), %esi
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movl oEBP(%eax), %ebp
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movl oEBX(%eax), %ebx
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- movl oEDX(%eax), %edx
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- movl oECX(%eax), %ecx
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- movl oEAX(%eax), %eax
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+
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+ /* All done, return 0 for success. */
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+ xorl %eax, %eax
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/* End FDE here, we fall into another context. */
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cfi_endproc
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diff --git a/sysdeps/unix/sysv/linux/i386/swapcontext.S b/sysdeps/unix/sysv/linux/i386/swapcontext.S
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index 431f22c..ce27d51 100644
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--- a/sysdeps/unix/sysv/linux/i386/swapcontext.S
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+++ b/sysdeps/unix/sysv/linux/i386/swapcontext.S
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@@ -26,13 +26,7 @@ ENTRY(__swapcontext)
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/* Load address of the context data structure we save in. */
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movl 4(%esp), %eax
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- /* Return value of swapcontext. EAX is the only register whose
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- value is not preserved. */
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- movl $0, oEAX(%eax)
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-
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- /* Save the 32-bit register values and the return address. */
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- movl %ecx, oECX(%eax)
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- movl %edx, oEDX(%eax)
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+ /* Save the preserved register values and the return address. */
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movl %edi, oEDI(%eax)
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movl %esi, oESI(%eax)
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movl %ebp, oEBP(%eax)
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@@ -91,15 +85,14 @@ ENTRY(__swapcontext)
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/* Push the return address on the new stack so we can return there. */
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pushl %ecx
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- /* Load the values of all the 32-bit registers (except ESP).
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- Since we are loading from EAX, it must be last. */
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+ /* Load the values of all the preserved registers (except ESP). */
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movl oEDI(%eax), %edi
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movl oESI(%eax), %esi
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movl oEBP(%eax), %ebp
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movl oEBX(%eax), %ebx
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- movl oEDX(%eax), %edx
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- movl oECX(%eax), %ecx
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- movl oEAX(%eax), %eax
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+
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+ /* All done, return 0 for success. */
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+ xorl %eax, %eax
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/* The following 'ret' will pop the address of the code and jump
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to it. */
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diff --git a/sysdeps/unix/sysv/linux/i386/ucontext_i.sym b/sysdeps/unix/sysv/linux/i386/ucontext_i.sym
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index b11a550..1dfe03d 100644
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--- a/sysdeps/unix/sysv/linux/i386/ucontext_i.sym
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+++ b/sysdeps/unix/sysv/linux/i386/ucontext_i.sym
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@@ -21,9 +21,6 @@ oESI mreg (ESI)
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oEBP mreg (EBP)
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oESP mreg (ESP)
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oEBX mreg (EBX)
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-oEDX mreg (EDX)
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-oECX mreg (ECX)
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-oEAX mreg (EAX)
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oEIP mreg (EIP)
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oFPREGS mcontext (fpregs)
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oSIGMASK ucontext (uc_sigmask)
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