From: Juergen Gross Date: Mon, 5 Jan 2026 08:04:51 +0000 (+0100) Subject: x86/alternative: Use helper functions for patching alternatives X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=544b4e15ed106b0e8cd2d584f576e3fda13d8f5f;p=thirdparty%2Fkernel%2Flinux.git x86/alternative: Use helper functions for patching alternatives Tidy up apply_alternatives() by moving the main patching action of a single alternative instance into 3 helper functions: - analyze_patch_site() for selection whether patching should occur or not and to handle nested alternatives. - prep_patch_site() for applying any needed relocations and issuing debug prints for the site. - patch_site() doing the real patching action, including optimization of any padding NOPs. In prep_patch_site() use __apply_relocation() instead of text_poke_apply_relocation(), as the NOP optimization is now done in patch_site() for all cases. Suggested-by: Borislav Petkov Signed-off-by: Juergen Gross Signed-off-by: Borislav Petkov (AMD) Link: https://patch.msgid.link/20260105080452.5064-2-jgross@suse.com --- diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 28518371d8bf3..6e3eec048d192 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -586,6 +586,88 @@ static inline u8 * instr_va(struct alt_instr *i) return (u8 *)&i->instr_offset + i->instr_offset; } +struct patch_site { + u8 *instr; + struct alt_instr *alt; + u8 buff[MAX_PATCH_LEN]; + u8 len; +}; + +static void __init_or_module analyze_patch_site(struct patch_site *ps, + struct alt_instr *start, + struct alt_instr *end) +{ + struct alt_instr *r; + + ps->instr = instr_va(start); + ps->len = start->instrlen; + + /* + * In case of nested ALTERNATIVE()s the outer alternative might add + * more padding. To ensure consistent patching find the max padding for + * all alt_instr entries for this site (nested alternatives result in + * consecutive entries). + */ + for (r = start+1; r < end && instr_va(r) == ps->instr; r++) { + ps->len = max(ps->len, r->instrlen); + start->instrlen = r->instrlen = ps->len; + } + + BUG_ON(ps->len > sizeof(ps->buff)); + BUG_ON(start->cpuid >= (NCAPINTS + NBUGINTS) * 32); + + /* + * Patch if either: + * - feature is present + * - feature not present but ALT_FLAG_NOT is set to mean, + * patch if feature is *NOT* present. + */ + if (!boot_cpu_has(start->cpuid) == !(start->flags & ALT_FLAG_NOT)) + ps->alt = NULL; + else + ps->alt = start; +} + +static void __init_or_module prep_patch_site(struct patch_site *ps) +{ + struct alt_instr *alt = ps->alt; + u8 buff_sz; + u8 *repl; + + if (!alt) { + /* Nothing to patch, use original instruction. */ + memcpy(ps->buff, ps->instr, ps->len); + return; + } + + repl = (u8 *)&alt->repl_offset + alt->repl_offset; + DPRINTK(ALT, "feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x", + alt->cpuid >> 5, alt->cpuid & 0x1f, + ps->instr, ps->instr, ps->len, + repl, alt->replacementlen, alt->flags); + + memcpy(ps->buff, repl, alt->replacementlen); + buff_sz = alt->replacementlen; + + if (alt->flags & ALT_FLAG_DIRECT_CALL) + buff_sz = alt_replace_call(ps->instr, ps->buff, alt); + + for (; buff_sz < ps->len; buff_sz++) + ps->buff[buff_sz] = 0x90; + + __apply_relocation(ps->buff, ps->instr, ps->len, repl, alt->replacementlen); + + DUMP_BYTES(ALT, ps->instr, ps->len, "%px: old_insn: ", ps->instr); + DUMP_BYTES(ALT, repl, alt->replacementlen, "%px: rpl_insn: ", repl); + DUMP_BYTES(ALT, ps->buff, ps->len, "%px: final_insn: ", ps->instr); +} + +static void __init_or_module patch_site(struct patch_site *ps) +{ + optimize_nops(ps->instr, ps->buff, ps->len); + text_poke_early(ps->instr, ps->buff, ps->len); +} + /* * Replace instructions with better alternatives for this CPU type. This runs * before SMP is initialized to avoid SMP problems with self modifying code. @@ -599,9 +681,7 @@ static inline u8 * instr_va(struct alt_instr *i) void __init_or_module noinline apply_alternatives(struct alt_instr *start, struct alt_instr *end) { - u8 insn_buff[MAX_PATCH_LEN]; - u8 *instr, *replacement; - struct alt_instr *a, *b; + struct alt_instr *a; DPRINTK(ALT, "alt table %px, -> %px", start, end); @@ -625,59 +705,11 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start, * order. */ for (a = start; a < end; a++) { - unsigned int insn_buff_sz = 0; - - /* - * In case of nested ALTERNATIVE()s the outer alternative might - * add more padding. To ensure consistent patching find the max - * padding for all alt_instr entries for this site (nested - * alternatives result in consecutive entries). - */ - for (b = a+1; b < end && instr_va(b) == instr_va(a); b++) { - u8 len = max(a->instrlen, b->instrlen); - a->instrlen = b->instrlen = len; - } - - instr = instr_va(a); - replacement = (u8 *)&a->repl_offset + a->repl_offset; - BUG_ON(a->instrlen > sizeof(insn_buff)); - BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32); - - /* - * Patch if either: - * - feature is present - * - feature not present but ALT_FLAG_NOT is set to mean, - * patch if feature is *NOT* present. - */ - if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) { - memcpy(insn_buff, instr, a->instrlen); - optimize_nops(instr, insn_buff, a->instrlen); - text_poke_early(instr, insn_buff, a->instrlen); - continue; - } - - DPRINTK(ALT, "feat: %d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x", - a->cpuid >> 5, - a->cpuid & 0x1f, - instr, instr, a->instrlen, - replacement, a->replacementlen, a->flags); - - memcpy(insn_buff, replacement, a->replacementlen); - insn_buff_sz = a->replacementlen; - - if (a->flags & ALT_FLAG_DIRECT_CALL) - insn_buff_sz = alt_replace_call(instr, insn_buff, a); - - for (; insn_buff_sz < a->instrlen; insn_buff_sz++) - insn_buff[insn_buff_sz] = 0x90; - - text_poke_apply_relocation(insn_buff, instr, a->instrlen, replacement, a->replacementlen); - - DUMP_BYTES(ALT, instr, a->instrlen, "%px: old_insn: ", instr); - DUMP_BYTES(ALT, replacement, a->replacementlen, "%px: rpl_insn: ", replacement); - DUMP_BYTES(ALT, insn_buff, insn_buff_sz, "%px: final_insn: ", instr); + struct patch_site ps; - text_poke_early(instr, insn_buff, insn_buff_sz); + analyze_patch_site(&ps, a, end); + prep_patch_site(&ps); + patch_site(&ps); } kasan_enable_current();