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objtool, xen: Fix INSN_SYSCALL / INSN_SYSRET semantics

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Objtool uses an arbitrary rule for INSN_SYSCALL and INSN_SYSRET that
almost works by accident: if it's in a function, control flow continues
after the instruction, otherwise it terminates.

That behavior should instead be based on the semantics of the underlying
instruction.  Change INSN_SYSCALL to always preserve control flow and
INSN_SYSRET to always terminate it.

The changed semantic for INSN_SYSCALL requires a tweak to the
!CONFIG_IA32_EMULATION version of xen_entry_SYSCALL_compat().  In Xen,
SYSCALL is a hypercall which usually returns.  But in this case it's a
hypercall to IRET which doesn't return.  Add UD2 to tell objtool to
terminate control flow, and to prevent undefined behavior at runtime.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com> # for the Xen part
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/19453dfe9a0431b7f016e9dc16d031cad3812a50.1744095216.git.jpoimboe@kernel.org
This commit is contained in:
Josh Poimboeuf
2025-04-08 00:02:16 -07:00
committed by Ingo Molnar
parent 9f9cc012c2
commit 2dbbca9be4
2 changed files with 15 additions and 12 deletions
Download Patch File Download Diff File Expand all files Collapse all files
tools/objtool/check.c
+14 -9
View File
@@ -3685,14 +3685,19 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
break;
case INSN_SYSCALL:
case INSN_SYSRET:
if (func) {
if (!next_insn || !next_insn->hint) {
WARN_INSN(insn, "unsupported instruction in callable function");
return 1;
}
break;
if (func && (!next_insn || !next_insn->hint)) {
WARN_INSN(insn, "unsupported instruction in callable function");
return 1;
}
break;
case INSN_SYSRET:
if (func && (!next_insn || !next_insn->hint)) {
WARN_INSN(insn, "unsupported instruction in callable function");
return 1;
}
return 0;
case INSN_STAC:
@@ -3888,9 +3893,9 @@ static int validate_unret(struct objtool_file *file, struct instruction *insn)
return 1;
case INSN_SYSCALL:
break;
case INSN_SYSRET:
if (insn_func(insn))
break;
return 0;
case INSN_NOP: