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Only shift unsigned values to avoid implementation-specific behavior.

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This converts the arithmetic shifts to logical shifts.
It is based in part on a stackoverflow answer by John Schultz,
https://stackoverflow.com/questions/4533076/google-protocol-buffers-zigzag-encoding
This commit is contained in:
Todd C. Miller 2022-06-06 13:57:38 -06:00
parent f224ab2eeb
commit 6e389ce2c3
1 changed files with 11 additions and 12 deletions
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23
protobuf-c/protobuf-c.c
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@ -316,9 +316,8 @@ int32_size(int32_t v)
static inline uint32_t
zigzag32(int32_t v)
{
// Note: the right-shift must be arithmetic
// Note: left shift must be unsigned because of overflow
return ((uint32_t)(v) << 1) ^ (uint32_t)(v >> 31);
// Note: Using unsigned types prevents undefined behavior
return ((uint32_t)v << 1) ^ -((uint32_t)v >> 31);
}
/**
@ -380,9 +379,8 @@ uint64_size(uint64_t v)
static inline uint64_t
zigzag64(int64_t v)
{
// Note: the right-shift must be arithmetic
// Note: left shift must be unsigned because of overflow
return ((uint64_t)(v) << 1) ^ (uint64_t)(v >> 63);
// Note: Using unsigned types prevents undefined behavior
return ((uint64_t)v << 1) ^ -((uint64_t)v >> 63);
}
/**
@ -802,7 +800,8 @@ uint32_pack(uint32_t value, uint8_t *out)
}
/**
* Pack a signed 32-bit integer and return the number of bytes written.
* Pack a signed 32-bit integer and return the number of bytes written,
* passed as unsigned to avoid implementation-specific behavior.
* Negative numbers are encoded as two's complement 64-bit integers.
*
* \param value
@ -813,14 +812,14 @@ uint32_pack(uint32_t value, uint8_t *out)
* Number of bytes written to `out`.
*/
static inline size_t
int32_pack(int32_t value, uint8_t *out)
int32_pack(uint32_t value, uint8_t *out)
{
if (value < 0) {
if ((int32_t)value < 0) {
out[0] = value | 0x80;
out[1] = (value >> 7) | 0x80;
out[2] = (value >> 14) | 0x80;
out[3] = (value >> 21) | 0x80;
out[4] = (value >> 28) | 0x80;
out[4] = (value >> 28) | 0xf0;
out[5] = out[6] = out[7] = out[8] = 0xff;
out[9] = 0x01;
return 10;
@ -2425,7 +2424,7 @@ static inline int32_t
unzigzag32(uint32_t v)
{
// Note: Using unsigned types prevents undefined behavior
return (int32_t)((v >> 1) ^ (~(v & 1) + 1));
return (int32_t)((v >> 1) ^ -(v & 1));
}
static inline uint32_t
@ -2467,7 +2466,7 @@ static inline int64_t
unzigzag64(uint64_t v)
{
// Note: Using unsigned types prevents undefined behavior
return (int64_t)((v >> 1) ^ (~(v & 1) + 1));
return (int64_t)((v >> 1) ^ -(v & 1));
}
static inline uint64_t