Messages in this thread Patch in this message | | | From | Juergen Gross <> | Subject | [PATCH v2 2/4] x86/alternative: add indirect call patching | Date | Mon, 16 Oct 2023 14:39:31 +0200 |
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In order to prepare replacing of paravirt patching with alternative patching, add the capability to replace an indirect call with a direct one to alternative patching.
This is done via a new flag ALT_FLAG_CALL as the target of the call instruction needs to be evaluated using the value of the location addressed by the indirect call. For convenience add a macro for a default call instruction. In case it is being used without the new flag being set, it will result in a BUG() when being executed. As in most cases the feature used will be X86_FEATURE_ALWAYS add another macro ALT_CALL_ALWAYS usable for the flags parameter of the ALTERNATIVE macros.
For a complete replacement handle the special cases of calling a nop function and an indirect call of NULL the same way as paravirt does.
Signed-off-by: Juergen Gross <jgross@suse.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/include/asm/alternative.h | 5 ++++ arch/x86/kernel/alternative.c | 40 ++++++++++++++++++++++++++++++ 2 files changed, 45 insertions(+)
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h index 67e50bd40bb8..dd63b96577e9 100644 --- a/arch/x86/include/asm/alternative.h +++ b/arch/x86/include/asm/alternative.h @@ -10,6 +10,9 @@ #define ALT_FLAG_NOT (1 << 0) #define ALT_NOT(feature) ((ALT_FLAG_NOT << ALT_FLAGS_SHIFT) | (feature)) +#define ALT_FLAG_CALL (1 << 1) +#define ALT_CALL(feature) ((ALT_FLAG_CALL << ALT_FLAGS_SHIFT) | (feature)) +#define ALT_CALL_ALWAYS ALT_CALL(X86_FEATURE_ALWAYS) #ifndef __ASSEMBLY__ @@ -150,6 +153,8 @@ static inline int alternatives_text_reserved(void *start, void *end) } #endif /* CONFIG_SMP */ +#define ALT_CALL_INSTR "call x86_BUG" + #define b_replacement(num) "664"#num #define e_replacement(num) "665"#num diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 1c8dd8e05f3f..01b89a10d219 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -395,6 +395,40 @@ noinstr void x86_BUG(void) } EXPORT_SYMBOL_GPL(x86_BUG); +/* + * Rewrite the "call x86_BUG" replacement to point to the target of the + * indirect pv_ops call "call *disp(%ip)". + */ +static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a) +{ + void *target, *bug = &x86_BUG; + + if (a->replacementlen != 5 || insn_buff[0] != CALL_INSN_OPCODE) { + pr_err("Alternative: ALT_FLAG_CALL set for a non-call replacement instruction\n"); + pr_err(" Ignoring the flag for the instruction at %pS (%px)\n", instr, instr); + return 5; + } + + if (a->instrlen != 6 || instr[0] != 0xff || instr[1] != 0x15) { + pr_err("Alternative: ALT_FLAG_CALL set for unrecognized indirect call\n"); + pr_err(" Not replacing the instruction at %pS (%px)\n", instr, instr); + return -1; + } + + /* ff 15 00 00 00 00 call *0x0(%rip) */ + target = *(void **)(instr + a->instrlen + *(s32 *)(instr + 2)); + if (!target) + target = bug; + + /* (x86_BUG - .) + (target - x86_BUG) := target - . */ + *(s32 *)(insn_buff + 1) += target - bug; + + if (target == &x86_nop) + return 0; + + return 5; +} + /* * Replace instructions with better alternatives for this CPU type. This runs * before SMP is initialized to avoid SMP problems with self modifying code. @@ -462,6 +496,12 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, memcpy(insn_buff, replacement, a->replacementlen); insn_buff_sz = a->replacementlen; + if (a->flags & ALT_FLAG_CALL) { + insn_buff_sz = alt_replace_call(instr, insn_buff, a); + if (insn_buff_sz < 0) + continue; + } + for (; insn_buff_sz < a->instrlen; insn_buff_sz++) insn_buff[insn_buff_sz] = 0x90; -- 2.35.3
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