d745852f1
Denys Vlasenko
2024-07-11
Subject
tls: P256: fix obscure x86_64 asm misbehavior, closes 15679
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networking/tls_sp_c32.c+29 -10
| @@ -425,26 +425,45 @@ static void sp_256_sub_8_p256_mod(sp_digit* r) | |
| #elif ALLOW_ASM && defined(__GNUC__) && defined(__x86_64__) | |
| static void sp_256_sub_8_p256_mod(sp_digit* r) | |
| { | |
| + | //p256_mod[3..0] = ffffffff00000001 0000000000000000 00000000ffffffff ffffffffffffffff |
| + | # if 0 |
| + | // gcc -Oz bug (?) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115875 |
| + | // uses buggy "push $-1; pop %rax" insns to load 00000000ffffffff |
| uint64_t reg; | |
| uint64_t ooff; | |
| - | //p256_mod[3..0] = ffffffff00000001 0000000000000000 00000000ffffffff ffffffffffffffff |
| asm volatile ( | |
| - | "\n addq $1, (%0)" // adding 1 is the same as subtracting ffffffffffffffff |
| - | "\n cmc" // only carry bit needs inverting |
| - | "\n" |
| - | "\n sbbq %1, 1*8(%0)" // %1 holds 00000000ffffffff |
| - | "\n" |
| + | "\n subq $0xffffffffffffffff, (%0)" |
| + | "\n sbbq %1, 1*8(%0)" |
| "\n sbbq $0, 2*8(%0)" | |
| - | "\n" |
| "\n movq 3*8(%0), %2" | |
| - | "\n sbbq $0, %2" // adding 00000000ffffffff (in %1) |
| - | "\n addq %1, %2" // is the same as subtracting ffffffff00000001 |
| + | "\n sbbq $0, %2" // subtract carry |
| + | "\n addq %1, %2" // adding 00000000ffffffff (in %1) |
| + | "\n" // is the same as subtracting ffffffff00000001 |
| "\n movq %2, 3*8(%0)" | |
| "\n" | |
| : "=r" (r), "=r" (ooff), "=r" (reg) | |
| - | : "0" (r), "1" (0x00000000ffffffff) |
| + | : "0" (r), "1" (0x00000000ffffffffUL) /* UL is important! */ |
| + | : "memory" |
| + | ); |
| + | # else // let's do it by hand: |
| + | uint64_t reg; |
| + | uint64_t rax; |
| + | asm volatile ( |
| + | "\n orl $0xffffffff, %%eax" // %1 (rax) = 00000000ffffffff |
| + | "\n subq $0xffffffffffffffff, (%0)" |
| + | "\n sbbq %1, 1*8(%0)" |
| + | "\n sbbq $0, 2*8(%0)" |
| + | "\n movq 3*8(%0), %2" |
| + | "\n sbbq $0, %2" // subtract carry |
| + | "\n addq %1, %2" // adding 00000000ffffffff (in %1) |
| + | "\n" // is the same as subtracting ffffffff00000001 |
| + | "\n movq %2, 3*8(%0)" |
| + | "\n" |
| + | : "=r" (r), "=&a" (rax), "=r" (reg) |
| + | : "0" (r) |
| : "memory" | |
| ); | |
| + | # endif |
| } | |
| #else | |
| static void sp_256_sub_8_p256_mod(sp_digit* r) | |