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X-OriginatorOrg: arm.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 May 2025 17:23:59.4155 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: 30e7fcc3-dac7-4964-b7fa-08dd899e203c X-MS-Exchange-CrossTenant-Id: f34e5979-57d9-4aaa-ad4d-b122a662184d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=f34e5979-57d9-4aaa-ad4d-b122a662184d;Ip=[4.158.2.129];Helo=[outbound-uk1.az.dlp.m.darktrace.com] X-MS-Exchange-CrossTenant-AuthSource: DB5PEPF00014B89.eurprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: DU0PR08MB10366 Hi Ard, > On Fri, 2 May 2025 at 16:58, Yeoreum Yun wrote: > > > > create_init_idmap() could be called before .bss section initialization > > which is done in early_map_kernel() since data/test_prot could be set > > wrong for PTE_MAYBE_NG macro. > > > > PTE_MAYBE_NG macro is set according to value of "arm64_use_ng_mappings". > > and this variable is located in .bss section. > > > > # llvm-objdump-21 --syms vmlinux-gcc | grep arm64_use_ng_mappings > > ffff800082f242a8 g O .bss 0000000000000001 arm64_use_ng_mappings > > > > If .bss section doesn't initialized, "arm64_use_ng_mappings" would be set > > with garbage value ant then the text_prot or data_prot could be set > > with garbgae value. > > > > Here is what i saw with kernel compiled via llvm-21 > > > > // create_init_idmap() > > ffff80008255c058: d10103ff sub sp, sp, #0x40 > > ffff80008255c05c: a9017bfd stp x29, x30, [sp, #0x10] > > ffff80008255c060: a90257f6 stp x22, x21, [sp, #0x20] > > ffff80008255c064: a9034ff4 stp x20, x19, [sp, #0x30] > > ffff80008255c068: 910043fd add x29, sp, #0x10 > > ffff80008255c06c: 90003fc8 adrp x8, 0xffff800082d54000 > > ffff80008255c070: d280e06a mov x10, #0x703 // =1795 > > ffff80008255c074: 91400409 add x9, x0, #0x1, lsl #12 // =0x1000 > > ffff80008255c078: 394a4108 ldrb w8, [x8, #0x290] ------------- (1) > > ffff80008255c07c: f2e00d0a movk x10, #0x68, lsl #48 > > ffff80008255c080: f90007e9 str x9, [sp, #0x8] > > ffff80008255c084: aa0103f3 mov x19, x1 > > ffff80008255c088: aa0003f4 mov x20, x0 > > ffff80008255c08c: 14000000 b 0xffff80008255c08c <__pi_create_init_idmap+0x34> > > ffff80008255c090: aa082d56 orr x22, x10, x8, lsl #11 -------- (2) > > > > Interesting. So Clang just shifts the value of "arm64_use_ng_mappings" > into bit #11, on the basis that 'bool' is a u8 that can only hold > values 0 or 1. > > It is actually kind of nice that this happened, or we would likely > have never found out - setting nG inadvertently on the initial ID map > is not something one would ever notice in practice. > ... Yeap. it's a quite nice and funny :D > > > > In case of gcc, according to value of arm64_use_ng_mappings (annoated as(3)), > > it branches to each prot settup code. > > However this is also problem since it branches according to garbage > > value too -- idmapping with wrong pgprot. > > > > I think the only way to deal with this in a robust manner is to never > call C code before clearing BSS. But this would mean clearing BSS > before setting up the ID map, which means it will run with the caches > disabled, making it slower and also making it necessary to perform > cache invalidation afterwards. > > Making arm64_use_ng_mappings __ro_after_init seems like a useful > change by itself, so I am not objecting to that. But we don't solve it > more fundamentally, please at least add a big fat comment why it is > important that the variable remains there. Agree. I'll add the comment on arm64_use_ng_mapping. Thanks! -- Sincerely, Yeoreum Yun